80/20 Training

In this Training article, we take an in depth look at the concept of 80/20 training.

There are a few ‘rules’ of training and racing that pop up in conversations between runners. For example:

  • Replace your shoes every 500 miles;
  • Increase your mileage by no more than 10% a week;
  • Negative split a race to run fast;
  • Long runs should be no more than a third of your weekly distance total;
  • Eat no later than three hours before running.

I’m not going to comment on these except to say they’re generally sound, but don’t necessarily apply to every runner in every situation.

Another so-called rule that gets repeated often is the ‘80/20 training‘ principle. Unlike some of the others, the evidence seems to suggest this is one rule almost always worth following.

In this article, we’ll explain what the 80/20 rule is, it’s benefits and limitations, and how to use it to improve your running.

The 80/20 what?

Simply put, the 80/20 rule of running training states that 80% of your weekly training time should be done at an easy effort level, with 20% consisting of harder running.

The distinction between easy and hard is based on the athlete’s ventilatory ‘threshold’. This is the point at which you switch from aerobic metabolism (using oxygen to fuel your muscles) to anaerobic metabolism (when there isn’t enough oxygen available for your muscles to use as fuel). It tends to occur at around 75-80 per cent of your maximum heart rate, but can vary for each runner. ‘Easy/slow’ running is below the threshold, and ‘hard/fast’ is close to, or above the threshold.

The theory is that as running is a predominantly aerobic activity, your training should reflect that with plenty of slower, easy-paced running, when you are best able to use your aerobic energy systems.

There is still a benefit to training your anaerobic energy systems because you will spend some time out of breath for that hard kick at the end of a race, or powering over a sharp hill – and because it helps raise the pace at which the threshold occurs. However, as distance runners, we don’t spend much of our races using our anaerobic energy systems, so it only needs to take up around a fifth of our training.

In addition, running slowly puts our muscles, bones and joints under less stress than faster running, so prioritising the easier paced runs reduces our risk of injury while still allowing for musculoskeletal adaptations.

The above might be an oversimplification, and glosses over the nuance of some of the science, but is, broadly speaking, what the 80/20 rule means.

Where has the rule come from?

Polarised training – combining easy training with hard training – has been practiced for decades. What’s relatively new is this theory that an 80/20 split between easy and hard is the specific sweet-spot to get the most benefit.

Dr Stephen Seiler, Professor in Sports Science at Norway’s University of Agder, is widely credited as the person who first highlighted the 80/20 rule in academic circles. It then became more widely known thanks, in part, to Matt Fitzgerald’s 2014 book, 80/20 Running.

Seiler would readily admit that though he came up with the term, ’80/20 training’, he didn’t invent it. Instead, he identified and explained something that élite endurance athletes, across a range of sports, were already doing – either instinctively or by design.

Whether it was top runners, pro cyclists, rowers, orienteers, or cross country skiers, Seiler noticed an overwhelming trend of the best endurance athletes in the world doing about 80 per cent of their training at very low intensities. He has since spent much of his career identifying just what it was about the 80/20 split that seemed to work so well.

What are the benefits?

In one study carried out by Seiler, with Iker Muñoz, Javier Bautista, Javier España, Eneko Larumbe and Jonathan Esteve-Lanao, a group of runners with 10K bests of around 40 minutes were split into different groups and given nine week training plans with either a 50/50 split, or an 80/20 split between easy and hard training. Both plans specified roughly 30 miles of running a week.

On average, the 50/50 group (which is the kind of training many recreational runners do) improved their 10K time trial times by 3.6 per cent. The 80/20 group improved by 5 per cent, which is equivalent to 35 seconds.

How can I do 80 20 training?

The great thing about this concept, is that it is scaleable, regardless of how much you run. Just split your training time up accordingly. Do 80 per cent of it an easy effort level, slow enough to maintain a conversation. Do the rest at a moderate or hard pace that leaves you out of breath.

Let’s consider some hypothetical example runners…


Adrian runs 5K, just once a week, taking about 30 minutes. He tries to push himself each time, so ends up doing all his runs at the same moderate to hard intensity.

Using the 80 20 principles, the most straightforward way Adrian could approach his weekly half hour run might be with a fast finish run: 24 minutes easy, 6 minutes hard.

Alternatively, by expanding his timeframe out, Adrian could do all his runs at an easy pace, except for every fifth week, when he runs as hard as he can. Over ten weeks, this would look like: easy, easyeasyeasy, flateasyeasyeasyeasy, flat, etc.


Bridget runs four times a week. She does 3 one hour runs on weekdays, plus a two hour long run at the weekend. Given that this equals five hours of running a week, it might appear reasonable for one of her weekday runs to be done as hard as she can, as this would represent twenty per cent of the time she spends running.

This could work, but it might not be the best way to approach this, because running for an hour as fast as you can is a tough undertaking.

Instead, Bridget structures her week like this:

  • Monday – 60 mins easy;
  • Tuesday – 60 mins, including a loose fartlek/intervals of 30x (30 secs hard, 30 secs easy);
  • Wednesday – Rest day;
  • Thursday – 60 mins, consisting of 45 mins easy, with 15 mins speeding up to finish;
  • Friday – Rest day;
  • Saturday – 2 hours, to be run at an easy pace, but including 6x long intervals of 5 mins at a moderate pace, spaced evenly throughout the run;
  • Sunday – Rest day.

This will give her a total of four hours of easy running, plus one hard, but split up into manageable chunks. The range of paces keeps things interesting for Bridget.


Charlie has been running for a long time, and does about 60 miles a week when training for half marathons and marathons. This takes them about 8 hours, split over 6 days of running and one rest day. Because Charlie is an experienced runner, the training load they can handle is much higher than

With this much running there is an almost limitless array of ways in which Charlie could structure their training week according to the 80/20 rule. This is just one example, taking into account Charlie’s need for longer hard efforts to suit their marathon goals:

  • Monday – 1 hour easy run;
  • Tuesday – 1 hour interval session, with 7x (3 mins hard, 1 min easy) plus warm up and cool down;
  • Wednesday – 90 mins medium long run at an easy pace;
  • Thursday – 1 hour, consisting of 10 mins easy, 20 mins hard, 5 mins easy, 20 mins hard, 5 mins easy;
  • Friday – Rest day, with no running.
  • Saturday – 1 hour, with 5x (2 min hard-ish hill efforts, jog back recoveries);
  • Sunday – 2.5 hours, mostly easy with the final 15 mins speeding up ever-so-slightly.

When totalled up, the easy running comes to 394 minutes, which is more than eighty per cent of the weekly running duration. 86 minutes, or less than twenty per cent of Charlie’s running time is spent at non-easy paces. The hard sections are all of varying durations, which allows Charlie to run at a variety of faster paces (from moderate to flat-out) for maximum training benefit, while rarely finishing a run completely worn-out.

How slow is slow?

To get the greatest benefit, it appears as though the easier, slower runs in your plan should be done at a pace that many runners might consider too slow to be of any use.

The subjects of Seiler’s initial observations included élite Kenyan runners. He was surprised to see that in many cases, their easy runs are done at paces as slow as 8 or 9 minute miles. When you consider these are runners who can knock out a marathon faster than 5 minute mile pace, this might seem surprising.

By running slowly, runners can still experience the endurance boosting benefits (increased capillarisation, improved bone and muscle strength, greater aerobic efficiency, etc.), that come from aerobic training, but at an intensity that doesn’t cause too much damage to their bodies. Then, when the time comes for the hard training sessions, they are fresh enough to push themselves harder than if they hadn’t taken it easy beforehand.

To be certain you are running slow enough on your easy days, pay attention to your breath and your heart rate. If you can speak in sentences, you’re probably at about the right speed (This will be zone 1 or 2 according to most heart rate training methods).

This may feel too slow at first, but trust the science!

Plus, you get to enjoy the added bonus of being able to better take in your surroundings, enjoy the run more, and wake up ready and raring to go the next day.

Are there any flaws?

The difficulty with any rule like this – where a formula has been determined through observation and scientific research – is that it is likely to be based on averages. The 80/20 split has been determined by taking the average results from a wide range of athletes. For some of those athletes, the optimum split could have been closer to 90/10, or 70/30. Therefore, there’s a risk that 80/20 training might not be the optimum mix for you.

However, in this case, the research seems to suggest that almost every runner will benefit from getting their training mix as close to 80/20 as possible.

In which case, the biggest issue might be working out how to make this work for you.

The 80/20 split should really be worked out based on training time. If you plan your running according to distance, rather than duration, then trying to work out the split can be fiddly. This is especially true if you tend to run by feel, rather than at precise paces. In this case, you can get reasonably close by working out eighty or twenty per cent of your total weekly mileage and dividing it up that way. It’ll be slightly off compared to doing it by time (as a fast mile takes less time than a slow mile), but it’s not a bad starting point.

If you want to keep things really simple, just make most of your runs easy, with every fifth run done at a hard effort. This isn’t perfect, as unless your runs are short enough for you to maintain a hard pace all the way, there’s a chance you’ll be slightly skimping on the faster paced work, but that’s probably safer than overcooking the speed-work.

The main difficulty with the 80/20 rule might be that it can be difficult to work out exactly what is meant by easy or hard, slow or fast, aerobic or anaerobic, sub-threshold or above-threshold. If you’ve done plenty of races, or have undertaken scientific testing, you can quantify this information in a very precise way, but this isn’t particularly accessible for everyone. Also, for new runners (who might be experiencing rapid gains in fitness compared to more experienced runners who are already closer to their potential) threshold pace can change significantly the more your running improves.

Fortunately, in the absence of a lab, resorting to subjective measures of running intensity can still be reasonably effective. To be certain you’re running aerobically, slow down until you can comfortably maintain a conversation while running. If you can’t get more than a couple of words out, then you’re almost certainly working anaerobically! Obviously, there’s a big range in between these two points, but the more you run, the more you’ll get a feel for whether you’re taking it easy or pushing the pace.

If in doubt, err on the side of caution and keep it really slow for your easy runs. That way you’ll know you’re definitely getting the required eighty per cent of easy running in. Plus, you’ll be fresh enough to push sufficiently hard for your fast runs.

Also, remember to think about the other training you do; not just running. If you do four hours of easy swimming a week, then one hour of hard running a week might be about right. That said, splitting both your swimming and running into eighty per cent easy with twenty per cent hard might be better.

How do I combine this with my existing training plan?

It wouldn’t be surprising – given how prevalent 80/20 training is – if your existing training plan is already divided up along these lines. Have a look at it, try and classify the runs as easy or hard running, and work out the split. If it’s heavily weighted in favour of fast running (twenty-five per cent or more), then consider whether you can adjust your plan to be closer to 80/20. The science that Seiler and colleagues have published would suggest that you’d most likely be improving your training plan by doing so.

If you’re a new runner, working their way through Couch to 5K, then your body is likely to still be getting used to the act of running. At the moment, sticking to just the one new training stimulus (increasing the amount of running you’re doing) is the safest way for you to progress without getting injured.

Adding faster running on top of the extra distance increases the risk of overloading your body as it adapts to your new sport. Don’t try and incorporate the 80/20 training principles now and stick to following the Couch to 5K plan. Once you’ve reached 5K, or 30 minutes of running, and you feel comfortable running for that length of time, you can start to add some speed to the mix.

Likewise, if you’re currently trying to increase your overall mileage from what you’re used to, then it might be sensible to temporarily reduce the percentage of faster running while you adjust to the extra impact of those additional miles. Build the distance first, then add the speed back in.

Shall we recap?

The 80/20 training rule, as identified by Dr Stephen Seiler, states that endurance athletes should do around eighty per cent of their training at a very easy intensity, with the remaining twenty per cent consisting of moderate or hard training. Scientific research suggests this is the optimum ratio for maximum training benefit and leads directly to improved race times or fitness gains.

It can be applied to all runners, of all abilities (except perhaps for brand new runners or those adjusting to increased mileage) as it can be scaled up or down depending on the amount of training you do. It can even take into account other sporting activities you do – not just running.

The 80/20 split can be achieved in a range of different ways, that may depend on whether the runner is focusing on shorter, faster events, or longer ones.

If you’re not sure of your precise training paces (ie: aerobic, threshold, anaerobic), you can get pretty close by subjectively classifying runs by how you feel: Easy would be conversational pace and could be slower than you currently run; threshold tends to be the maximum effort you can maintain for up to an hour; hard is anything faster, right up to pure sprinting.

If your running is already organised according to 80/20 principles, then you can crack on with your training, reassured by the knowledge you’re getting what seems to be the optimum mix of training intensities.

If you’re not yet training in this way, you can feel enthused by the thought that with a few easy tweaks, you could potentially unlock significant fitness gains or improvements in your race times.

Anything else?

The focus of this article has been about optimising your training to improve your fitness, speed or race times. This is all based on the assumption that, if you’re reading an article about 80/20 training, these are goals of yours. If so, the advice above may help you achieve those goals.

However, we also recognise and celebrate that the great thing about running is you can do it however you want. There are as many different ways to run as there are runners.

If you’re passionate about high intensity intervals and sprint sessions, then go for it and do your thing!

On the other hand, if your idea of heaven is keeping it easy and taking in the sights, sounds and smells of your local environment, then by all means continue doing so.

Arguably, far more important than getting your mix of training intensities just right, is making sure that you are enjoying your running.


All are welcome at Lonely Goat Running Club, as it is the shared experience of thousands of different people that makes it such a special, supportive community. Sign up for a free account and engage with the club features that suit you best.

For more training articles, click on this link.

Support the club so we can do bigger and better things and get access to additional benefits.
£25 per year