Fartlek, tempo and intervals: which hard run does what

Fartlek is the only running workout with a name that makes people giggle and a structure that makes people faster. It's Swedish for "speed play", and that's the whole idea: run hard for a bit, run easy for a bit, repeat, no track required.

It's also one of three hard runs that get mixed up constantly. Fartlek, tempo and intervals all involve running faster than usual, but they do different jobs. Do the wrong one at the wrong time and you get a week of hard running that doesn't build what you need. Here's what a fartlek run is, how fartlek training compares with tempo and intervals, and which one belongs in your week, whether you plan it yourself or follow a running coach in your pocket.

What is a fartlek run?

A fartlek run is a continuous run where you alternate faster and slower running without stopping. The surges can be timed ("2 minutes hard, 2 minutes easy") or completely unstructured ("hard to that lamp post, easy until I've recovered").

It has a real history. According to Athletics Weekly, fartlek was developed in the 1930s by Swedish coach Gösta Holmér, a former Olympic decathlon medalist, as a response to the dominance of Finnish distance runners. The original version was loose: varied intensity, varied terrain, running through the countryside. As one coach quoted in that piece puts it, a fartlek can be the easiest session of your week or the hardest, depending on how you run it.

That flexibility is why fartlek shows up in almost every kind of plan, from a first 5K to a 12-week half marathon plan. It's the friendliest way to start running fast, and it's still useful for experienced runners who want quality without staring at a watch.

Fartlek vs tempo vs intervals

All three are "hard runs". They hit different intensities and train different things. Using the five HR zones we use across this blog:

The quick version:

• ›Tempo raises your threshold. It teaches you to hold a hard pace for a long time. If you want to understand that number, start with how to test lactate threshold.
• ›Intervals raise your ceiling. In Helgerud et al. (2007), moderately trained men who did 4 × 4-minute running intervals at 90–95% of max heart rate improved VO2 max more than groups doing the same total work at threshold (85% of max) or at an easy 70%.
• ›Fartlek does a bit of both, with less pressure. The surges can be threshold-ish or interval-ish depending on how long and hard you make them.

Tempo runs

A tempo run is a sustained, "comfortably hard" effort: roughly the pace you could race for about an hour, in common coaching terms. It sits in zone 4 at around 80–90% of max heart rate. You can say a few words, not sentences. Classic forms are a continuous 20–30 minutes at that effort, or the same total time split into long reps with short jogs.

Tempo is controlled. If you finish a tempo run wrecked, you ran an interval session by accident.

Interval runs

Intervals are short to medium hard reps, from about 30 seconds up to a few minutes, with recovery between them. The hard reps live in zone 5, at RPE 8–9. Heart rate lags behind on short reps, so you judge intervals by pace or by RPE-based effort rather than by heart rate. Interval training for runners ranges from short, fast reps for speed to longer reps for VO2 max.

Fartlek runs

Fartlek sits in between. It's continuous like tempo, varied like intervals, and flexible enough to be either. The recoveries are run, not walked or stood, which keeps the session aerobic. And because there's no prescribed pace, it's excellent for hills, trails, bad weather and days when you don't want to be bullied by a watch.

Fartlek workouts to try

All of these start with 10–15 minutes easy and end with 10 minutes easy.

Rules that keep fartlek useful:

• ›The easy parts are actually easy. If you're surging and then "recovering" at tempo pace, it's just a tempo run with mood swings.
• ›Start conservative. The first surge should feel too easy. The last should feel hard but repeatable.
• ›Finish feeling like you could do one more. That's the point of play, not punishment.

Which hard run do you need?

Pick based on your goal and where you are in your training.

• ›New to faster running: start with fartlek. Low pressure, no track, and you learn what different efforts feel like.
• ›Training for a 5K or 10K: a mix of intervals and tempo, with fartlek on weeks you want variety. 5K training usually leans more on intervals, since the race itself is short and fast. Fartlek is useful whether you're chasing a PR or just figuring out how fast a 5K should be for you.
• ›Training for a half or full marathon: more tempo and threshold work, intervals in smaller doses. For longer races, interval training for distance runners tends toward longer reps, and track workouts for marathon training lean toward volume at controlled paces.
• ›Hybrid or Hyrox-focused: one hard run a week is often enough while lifting is the priority. Fartlek is a good default because it's flexible and easy to scale on a tired day.

Most runners do well with one or two hard runs a week. Not three. Not every day.

The other runs matter more than you think

Hard runs get all the attention. But most of a good running week is easy. Well-trained endurance athletes do roughly 80% of their sessions at low intensity and about 20% hard (Seiler, 2010). That's 80/20 running and polarized training, explained in a sentence. In a nine-week trial in well-trained athletes, a polarized program, lots of easy work plus some very hard sessions, improved key endurance markers more than threshold-focused or high-volume programs (Stöggl and Sperlich, 2014).

That means your hard sessions only work if your easy runs stay easy. And if you've ever asked what is an easy run, the answer is almost always: slower than the pace you're running now, conversational, zone 2. Add the long run, also easy, and you have the full picture: mostly easy, a long run, one or two hard runs.

A sample week with lifting:

The two hard runs are separated, and the heavy leg day sits on an easy run day with rest after it. That's the logic of balancing runs and lifts: keep the hard things apart, let the easy things fill in.

Beyond fartlek: sprints and the track

Once fartlek feels comfortable, two natural next steps:

• ›Sprints: very short, fast efforts (often 10–20 seconds) with full recovery. Sprint training for beginners starts with strides at the end of easy runs before anything more intense.
• ›The track: measured distances make intervals precise. Track workouts for beginners usually start with something like 400 m or 800 m repeats, run at controlled effort.

Neither is required. Plenty of runners get fit on fartlek, tempo and easy miles alone.

Common mistakes with hard runs

• ›Every run is a hard run. If your easy days are RPE 6, your hard days can't be hard enough.
• ›Racing the workout. Intervals and tempo have a target. Beating it by a mile today means paying for it tomorrow.
• ›Doing intervals the day after heavy squats. Your legs know what you did.
• ›Never changing anything. The same fartlek every week becomes a habit, not a stimulus. Progress the reps, the length, or the effort.

How exrsz plans hard runs

exrsz builds a periodized plan from your goal, schedule, level and equipment, choosing fartlek, tempo or intervals based on the phase you're in and the race you're training for. It balances those hard runs against your lifting in one plan, adapts each week to how you actually trained, and reshuffles missed sessions so you don't end up stacking two hard runs together.

FAQ

What is a fartlek run?

A fartlek run is a continuous run that alternates faster surges with easier running, without stopping. The name is Swedish for "speed play".

What is the difference between fartlek and interval training?

Intervals are structured hard reps with set recoveries, usually at very high intensity. Fartlek is continuous, more flexible, and the surges can be anywhere from moderate to very hard.

Is a fartlek run better than a tempo run?

Neither is better; they do different jobs. Tempo builds your lactate threshold with sustained effort, while fartlek mixes efforts and is easier to adapt to terrain and how you feel.

How often should I do fartlek training?

Once a week is plenty for most runners, as one of one or two hard runs. Keep the rest of the week mostly easy.

Is fartlek good for beginners?

Yes. A short fartlek with gentle surges is one of the easiest ways to start running faster, because there's no pace target and you can adjust each surge to how you feel.