Training: Intervals & Fartleks

Do you feel the need for speed? Then try adding some speedwork to your running training: Intervals and Fartleks – pushing the pace in training to improve your running.

Run faster to run faster

Improvement in running is the product of a balanced training plan that covers all the energy systems and physiological requirements relevant to your goals in your chosen event. This requires speed, as well as endurance.

Many runners only focus on their endurance – by increasing their distances – and neglect to do any faster running. They then find they can keep going for ages, but cannot speed up. The solution to this is simple: If you want to run faster, you have to run faster.

Fast twitch and slow twitch

Our muscle fibres can broadly be categorised into two groups: slow twitch or fast twitch. The differences are complex, but to generalise, slow twitch fibres don’t contract as quickly, but can keep going for longer. Fast twitch contract very quickly, but only for short bursts.

When running slowly, our bodies predominantly use our slow twitch fibres as they’re more efficient for moderate efforts over a long period of time. By the end of a long race, however, the muscle damage may be so great that a lot of the slow twitch fibres are not at their best, requiring us to call on the fast twitch fibres to help out.

Research suggests that if we’ve used our fast twitch fibres in training, by sprinting, our bodies will become more comfortable with cycling them in and out – even when doing slower running – to reduce the burden on the slow twitch fibres.

This is an oversimplification, as it’s a big subject to go into here, but the general principle is that if we’ve used the muscles in training, we’ll be better able to use them when racing.

Biomechanical benefits

Some runners spend time at the gym doing exercises that resemble actions performed when running: leg extensions, calf raises, squats, core work, etc. These can be beneficial, but it is worth remembering that no exercise will more closely resemble running than running itself.

Running is a compound exercise (one involving multiple muscle groups performing different movements), in which the athlete does around 150-200 low intensity repetitions every minute, for minutes at a time. It is hard to replicate that in a gym.

Sprinting is a great way of taking that compound exercise and increasing the intensity. You are likely to perform more reps per minute, and move your muscles through a wider range of motion when sprinting.

If you sprint uphill, you then add a further element by introducing more resistance as you have to overcome gravity, too.

How to add speed

To an extent, if you are currently doing no speedwork whatsoever, then anything you add is likely to yield an improvement. The difficult thing is working out what speedwork you should do and how to fit it in to the rest of your plan, as there is no set formula or single best way of doing it.

That said, for long distance runners, speedwork can usually be classed as either intervals or fartleks.


These are sessions where you run at a hard pace for a set distance or time, stop or slow down to recover, then repeat the process.

There are a practically infinite number of ways this can be done, depending on the number of repetitions (the fast sections, often abbreviated to reps), length of the reps, speed of the reps, and the amount of recovery in between.

At the start of a training plan, keep things manageable with fewer, or shorter reps, with long recoveries. Over the course of a training plan you can then make them progressively harder as you get fitter.

A good target to aim at is being able to cover the race distance or longer at the goal pace or quicker, with as few breaks as possible.


Interval sessions are fairly structured, but it is also possible to run unstructured sessions, known as fartleks (a Swedish word meaning “speed play”). These can take whatever form you like, with faster bits of running broken up with slower bits. The pace doesn’t have to be precise either as you can run by feel – trying to maintain a high intensity without worrying too much about the exact speed.

Example fartlek sessions include:

  • Using lampposts as markers – speeding up every time you pass a lamppost, then slowing when you pass another;
  • Sprinting to the next street corner every time you see a car of a particular colour; or
  • Using other things on your run as markers or prompts to put in a burst of speed.

This can be a fun way of introducing speedwork to your training without the stress that can come from being too rigid with distances and paces. The unpredictability can be beneficial to experienced runners too, as it can break you out of your normal habits and provide a different stimulus.

Lonely Goats, Gilly Salmoni and Katryne Cook
How to incorporate speedwork into your training

As with all parts of a training plan, you need to consider what you are trying to achieve, what it will take to do so, and tailor that approach according to your circumstances. To illustrate this, let’s consider the following example runners.

Andrea – looking to run her first marathon

Her main focus is, understandably, her long runs, but speed shouldn’t be neglected. Andrea knows that by the end of the marathon she’ll be very tired and her running form is likely to go to pieces.

Andrea doesn’t need to do too much speedwork, as marathons are mostly slow twitch affairs, but one session a week with half a dozen short sprints up a gradual hill, and long “walk-back” recoveries between sprints may be sufficient for her. A little bit of occasional fartlek work could help too.

By doing hill sprints – which build muscular strength – throughout her plan she may be able to retain her form for longer. Also, the sprinting will develop her ability to recruit all the muscle fibres she needs in the marathon.

Baz – beginner doing Couch to 5K

Baz will benefit from speedwork for much the same reason as Andrea: to condition his body and prepare his muscles for running.

A word of caution, however: The high intensity nature of sprinting can cause muscle strains and other problems if you’re not used to it or warmed up properly, so always ease into any sprinting session and take it easy or stop if something hurts.

Claudia – training to run a fast mile

Claudia’s speedwork will look a little different to Andrea and Baz’s. She needs to prepare herself for the challenge of running as fast as possible for a mile. This will require getting used to running at mile race pace. This is a hard pace to run at for an extended period of time, but Claudia will be able to build up the duration she can manage by running interval sessions.

An example interval session progression

Using Claudia as an example, this is how you can incorporate interval sessions into a training plan.

Claudia is able to do her speedwork on a 400m long athletics track (a loop in the park would work too – a track isn’t essential – but it does make the maths easier for this example). With this in mind, her interval sessions over 7 weeks prior to tapering may be as follows:

  • 6 x 200m run at her target race pace, with 60 second recoveries (walking around to stay loose and letting her get her breath back);
  • 8 x 200m with 60 second recoveries;
  • 6 x 300m with 60 second recoveries;
  • 5 x 400m with 90 second recoveries;
  • 3 x 600m with 2 minute recoveries;
  • 4 x 500m with 60 second recoveries;
  • 2 x 800m with 90 second recovery;

The last session will be really hard, and Claudia wouldn’t be able to do it at the start of her training plan. However, by starting with something more manageable, with shorter reps, she can build up as her fitness improves.

Changing the variables

These are the three ways in which speedwork sessios can be made progressively harder over the course of a training plan:

  • By extending the length of each rep;
  • By increasing the speed of each rep;
  • By reducing the recovery between each rep.

It doesn’t matter which method you choose, but you may have an idea which option you are likely to benefit from the most based on your running history.

The option you like the look of the most is probably the one you would find easier. On the one hand, that’s good, as you are likely to get the hang of the session quickly, which can be motivating. On the other hand, it can be better to do the sessions you find hardest and don’t look forward to, going outside of your comfort zone and developing skills you might otherwise neglect.

Pros and cons

What are the advantages and disadvantages of intervals and speedwork?

  • Improves speed;
  • Builds strength;
  • Improves form.
  • Can lead to injury or fatigue;
  • An overemphasis on speed can lead to endurance being neglected.

What alternatives are there to intervals? The usual cross training options apply: Swimming, cycling, and the elliptical trainer in the gym. Particularly effective is simulating your running sessions on an indoor rower. For example, if your running training plan calls for 10x 2 minute hard efforts, you can do the exact same session when rowing.

Lonely Goat, Lewys Farnham
Run fast

Speedwork – be it unstructured fartleks, or structured interval sessions – is an essential part of every runner’s training plan. If you’ve got limited experience of running at a faster pace, then it can be useful to hold back slightly at the beginning while your body adapts to the change in intensity. Once you’ve gotten used to it though, the jumps in your running performance can be considerable.

There’s a large community of Lonely Goats who can help answer any questions you may have about adding speed to your run. Head over to our Facebook Chat Group or Strava Club and join the discussion.

The Lonely Goat running training series

Check out the rest in this series of training articles…

…plus our other articles on different aspects of running training.

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