Words by Laura Markwardt. Photography by Frankie Carino and Lea Kurth, Colin Wong (portraits).

If you’re new to running, it’s a good idea to focus on slowly building your endurance, and a solid running technique. But when you’re ready to up the pace, interval training can reap big rewards. If you’ve used the Couch-to-5K method to get started, you might be familiar with interval training already – blending consecutive blocks of high effort with intervals of rest.

Interval training works as a fast-track method to build strength, add speed and boost endurance, also taking experienced runners to the next level – and a new personal best.

“Besides easy runs and long runs, running intervals is the biggest part of my training,” says German track star, and three-time 800m National Indoor Champion, Robert Farken. 

But as George Beamish, the On Athletics Club (OAC) middle-distance champion who won 1,500m gold at the 2024 World Indoor Championships, advises, “Intervals should be one of the last additions to an amateur training plan. There are so many gains to be made from just easy running, tempo runs, hill sprints, and long runs before adding in hard intervals.” 

Known to his OAC contemporaries as “Textbook George,” because of his consistent kick on the home straight, Beamish has mastered the art of getting maximum gains from all-out sprints, strategically employed to maximum effect. When you’re ready to introduce interval training into your plan, here’s how Beamish and Farken recommend you begin.

What is interval training? 

Interval training tests your limits at high speeds but in short efforts. Each hard effort is followed by an interval rest period – this can be ‘active rest’ such as walking, and then you go again. Interval training is typically carried out in blocks or sessions of effort that can increase in intensity over time.

Interval training can deliver huge performance payoffs in a long-term training plan, but it’s important to conserve your energy if you’re wrapping up a training block closer to race day. 

As Beamish notes, “There’s a skillset around interval training—understanding the level of intensity you need to go at without depleting your body. Some people treat intervals as a race and have nothing left on race day.”

Sometimes referred to as HIIT (high-intensity interval training), interval training can be pure running, but it can also refer to intense sets of other exercises, such as mountain climbers, burpees, or press-ups. Even the most accomplished athletes don't run fast every time they train – the highest volume of your training should be at an easy pace. “Fartlek” training, from the Swedish meaning “speed play,” is also worth a mention here. Fartleks incorporate intervals into your run with no fixed structure – think sprinting to the next tree, the next traffic sign, etc.

Why should I run intervals? 

Once you can maintain a comfortable pace, intervals are a proven way to level up your running – and your average Strava pace – over a matter of weeks. Shorter and more varied in pace than the traditional long run, some runners even prefer intervals to slower sessions, embracing the intensity, efficiency and focus it takes to go all-out. 

Farken’s a fan. “When you’re interval training, the sessions don’t feel as long. It feels like you’re doing more in less time. For example, five lots of eight minutes is already 40 minutes of work at a high pace. Add a warm-up and cool-down and you’ve already got a big session under your belt. That can feel easier than running for an hour straight.”

Compared with continuous endurance exercise, interval training better increases your VO2 max, which is the amount of oxygen your body can use during intense exercise. If you’re looking to get fitter, increasing VO2 max is key – the more oxygen your body can process, the faster and further you can go before you feel exhausted.

Among other benefits, interval training is more effective than steady running for increasing the volume of blood your heart pumps when you’re approaching maximum exertion.

“I enjoy intervals. They’re a change of pace in terms of training, and offer a different kind of challenge than long, continuous runs,” says Farken. Think of it as upgrading your engine; a turbo-boost for the track or trail. The enhancements extend to the muscles too. 

Intervals are effective at increasing the size and number of mitochondria in your muscle cells. These tiny superheroes are the energy factories that help your muscles produce greater force for a longer period, making sure you’ve always got a kick for a sprint finish.

How do I start interval training? 

A solid interval session needs little more than some old-fashioned determination. As Beamish advises, “If you're just getting back into running, I would stick to basic tempo and hill sessions to begin with.”

Farken says the most important thing is to, “trust your training.” 

“Don’t just look at average pace at the end of your session, but focus on your individual target areas, where your training’s most effective.”

 Interval training with other runners 

Training with others to keep your effort levels high, sounds like a win. But, from experience of training with other On athletes, Farken emphasizes that it’s also important to admit if a training partner runs more efficiently at a higher intensity than you. “It might be frustrating, and you also want to be on that level. But you’ll only reach that bar if you work within your boundaries effectively. It takes time. Only then will you get better. Working too hard, too early, won’t make you a better runner,” says Farken. “You have to develop a sense of trust [with yourself], to stay within your boundaries and improve that way.”

“Don’t compare times and splits to others. That’s a piece of advice that everybody needs,” he continues. “Perhaps when you’re working out with someone, you’ll tell yourself ‘Come on, I’ll just keep up with them, and then you end up overexerting yourself.’ It’s important to admit to yourself that you’re going too hard, and take it a bit easier.”

“But I had one session with Tom Elmer where I thought ‘Definitely no, I’m out,’” Farken laughs. “When he said I only needed to keep up for two more intervals, I pushed myself, and we carried each other through the session. It’s cool to be honest when you’re in over your head.”

“It’s also cool to be just as honest if you’re feeling great, and you want to increase the pace further. But you have to communicate that rather than creating a false sense of competition in training. It’s important that everybody’s in on the plan – that you all agree on the purpose of the session.”

What should I wear for interval training?

In terms of kit, you want gear that’ll keep you at your comfortable best both in the heat of the sprints and the cooler breaks in between. The On Performance Long-T is ideal. It’s made to adapt to your body and the conditions, so you can focus on smashing out results. The On Weather Vest works well over the top as an outer layer that’s light enough to keep you at top speed during the intervals, but warm during recoveries.

Reactive shoes, built for pace and optimal support work best for intervals. Experienced runners might choose the Cloudboom Strike with a carbon plate for the fastest efforts and a cushioned landing. For an entry-level shoe, with a smooth, faster feeling, go for the Cloudsurfer Next.

You can run intervals on the road or trail, or even throw in some hills, but many runners choose the track. A full 400-meter loop is a common interval set, so with a track you can see where you’re up to as you’re chasing down the distances. Beamish points out, “Hills are really easy to incorporate into a training plan. Running hard up a hill for 10 to 20 seconds, six to eight times at the end of an easy run, will give you big benefits. Hills are always low risk [of injury], high reward training additions.”

You can divide your intervals by time or distance, so a GPS running watch is useful. If you use a heart-rate monitor, you can vary intervals based on your heart rate zone, too. Beamish explains, “I don’t run any intentional Zone 2 training generally. We focus on running on feel and what your body tells you is the right pace between the hard days of running.”

If you’re new to intervals, 8 x 400 meters with the same distance as rest in-between is a good place to start. As you improve, you can increase the distances or shorten the recovery time. Recovery is usually active rather than full rest, transitioning from all-out exertion to an easy jog. Most importantly, check that your recovery is gentle enough that you feel ready to go again when your watch tells you it’s time.

Interval sessions for getting started

1K warm-up easy pace, 400m fast, 30 seconds active rest > Repeat 8x 1K cool down, easy pace

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1K warm-up easy pace, 600m fast, 30 seconds active rest > Repeat 6x 1K cool down, easy pace

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1K warm-up easy pace, 800m fast, 30 seconds active rest > Repeat 5x 1K cool down, easy pace

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To conclude, interval training can deliver gains where they’re needed most – and look pretty impressive when you turn on that kick as you approach the finish line. It’s a simple formula that promises big results. 

Beamish on race day is a case in point. “My go-to interval session would be 1k reps,” he says. 

“Short but really fast intervals that are closer to race pace are my favorite sessions,” says Farken. “They can help you to assess where your fitness is, and what you might be able to produce in a race setting.”