What are intervals? A plain-English guide

What an interval session actually looks like

An interval session has four parts. First a warm-up: 10–15 minutes of easy running with a few relaxed strides (short, gentle pick-ups) to wake your legs up. Then the intervals themselves — a set number of harder efforts, called reps, each followed by a recovery jog. Then a cool-down of easy running to finish.

Written down, it looks like this: 6 × 3 minutes hard, 2 minutes easy jog. The first number is how many reps. The second is how long each rep lasts. The third is your recovery. You can measure reps by time (3 minutes) or by distance (1 km), and it doesn't matter much which you pick.

The recovery jog is part of the session, not a failure. Jog it slowly enough that you get your breath back before the next rep begins.

Why runners use them

Easy running builds your aerobic engine: your heart, lungs and muscles getting better at using oxygen over a long time. Intervals add something easy running can't — time spent running fast.

Running hard for a few minutes at a time pushes your heart to pump more blood with each beat and teaches your muscles to use oxygen quickly. Runners call the upper limit of this VO₂ max: the most oxygen your body can use in a minute. Raising it makes every other pace feel more comfortable. The same easy run feels easier, and race pace feels less desperate.

They also give you practice at running quickly with good form, which is a skill in itself rather than just a way to get fitter.

How hard should intervals feel?

Hard. Taper's own description is "hard · only a word or two at a time" — you can say a couple of words, not a full sentence. That's a notch above a tempo run, which is a sustained "comfortably hard" effort where short phrases are possible.

It should not feel like a sprint. Out of 10, aim for about 8. The first rep should feel almost too easy; the last one should be the hardest. If you're falling apart on rep two, you went out too fast. Start the first rep slower than you think you need to and let the later ones be the quickest.

A few common kinds

• Short reps on a track or flat path: 8–12 × 400 m hard with 200 m of easy jog between. Good for leg speed.
• Time-based reps anywhere: 5 × 3 minutes hard with 2 minutes of jogging. Easy to slot into a normal run.
• Hill repeats: run up a moderate hill for 60–90 seconds, jog down easy, repeat. Speedwork in disguise — it builds leg strength with less impact than flat fast running.
• Fartlek (Swedish for "speed play"): unstructured surges mixed into a run — to that tree, that lamppost — then ease back.

How to start, and how often

If you've never done them, start small: 4 × 1 minute faster with 2 minutes of walking or easy jogging in between, in the middle of a normal easy run. That counts as an interval session. Add a rep or a minute every week or two rather than adding extra sessions.

Once a week is a sensible starting point, and plenty of runners build real fitness on exactly that. The rest of your week is easy running, which is where most of your fitness comes from. Intervals are the seasoning, not the meal.

Leave at least 48 hours between hard sessions, keep intervals away from your long-run day, and don't stack them on top of a week where your legs are already tired. If something hurts in a way that changes how you run, stop and see a GP or physio.