Fartlek: the free-form run, explained and structured on your watch
Fartlek is a continuous run in which you alternate fast and easy stretches without ever stopping. The word is Swedish for "speed play". Here is what sets it apart from classic intervals, four formats with their paces, where it fits in your week, and how to get it on your Garmin watch with Running Workout.
Last updated: 2026-10-07
What is fartlek?
Fartlek is a continuous run built on pace changes: you speed up, then ease off into a jog, without ever walking or stopping. Unlike track intervals, the recovery is still running, and the session can follow the terrain rather than a stopwatch: a surge to the next bend, a hill run hard, a gentle slope jogged back down.
There are two ways to run it. Free fartlek is run by feel: you decide on the spot how long and how hard each surge is. Structured fartlek sets durations and paces in advance, like the 30/30 (30 seconds fast, 30 seconds easy). The first trains you to listen to your body; the second can be measured and compared from week to week.
Try it with your own Garmin
Free, nothing to install: connect your Garmin, then ask Claude or ChatGPT for your first workout. It lands on your watch within a minute.
Four fartlek formats
The four formats below range from the freest to the longest. The MAS percentages are guidelines; the 30/30 at 100% of MAS is the one used in Running Workout's 5K, 10K, half marathon and marathon plans, with 6 to 12 reps depending on the week.
| Format | Content | Pace | What for |
|---|---|---|---|
| Free | 30 to 40 min with 6 to 10 surges of 20 s to 3 min, chosen according to the terrain | By feel, from 10K pace to fast | Variety, running without staring at the watch |
| 30/30 | 8 to 12 × (30 s fast, 30 s jog) | 100% of MAS | Getting back in touch with speed |
| 2 min / 1 min | 6 to 8 × (2 min fast, 1 min jog) | About 90% of MAS, close to 10K pace | Training for a 5K or 10K |
| Long | 4 to 6 × (5 min steady-hard, 2 min jog) | 80 to 85% of MAS, between half marathon and threshold pace | Training for a half or full marathon |
Allow a 15-minute easy warm-up before and a 10-minute cool-down after. In every format, the recovery is a jog: that is what makes fartlek a continuous run rather than intervals with stops.
Paces for a MAS of 9.9 mph (16 km/h)
With a MAS of 9.9 mph (16 km/h), the fast parts of a structured fartlek are run between 6:02 and 7:33 per mile, depending on the format. For a different MAS, Running Workout's pace calculator gives the same benchmarks.
| Format | % MAS | Pace | Speed |
|---|---|---|---|
| 30/30 | 100% | 6:02 /mi | 9.9 mph |
| 2 min / 1 min | 90% | 6:42 /mi | 8.9 mph |
| Long | 80 to 85% | 7:06 to 7:33 /mi | 8.0 to 8.5 mph |
On a 30/30, 30 seconds is too short for the pace shown on the watch to settle: rely instead on consistency from one rep to the next. The last surge should look like the first; if it is clearly slower, the session was too long or started too fast.
Where fartlek fits in your week
A structured fartlek is a quality session: it replaces an interval session in your week rather than adding to it. Keep at least one easy day between it and another hard session or your long run.
- Returning to training or early in a build: the 30/30, short and not very tiring, to find your speed again.
- Mid-build: the 2 min / 1 min for the 5K and 10K, the long format for the half and full marathon.
- Recovery week or a period with no goal race: free fartlek, to keep some rhythm without chasing times.
- First time: start with 6 × 30/30 and add two reps a week if all goes well.
Structuring fartlek on your Garmin watch
With Running Workout, your connected AI assistant (Claude, ChatGPT or Gemini) builds the fartlek as a structured workout: a block of repeats, each part with its duration and a pace target calculated from your MAS, then sends it to Garmin Connect, scheduled for the date you choose if you give one. The watch beeps at every change: you just follow along.
Example request: "Create a fartlek for Saturday: 10 × 30/30, 30 seconds at 100% of MAS and 30 seconds of jogging, with a 15-minute warm-up and a 10-minute cool-down."
Free fartlek works on the watch too: the assistant can create a step with no fixed duration and no target, which you end yourself by pressing the Lap key. If Running Workout does not know your MAS, creating a workout in % of MAS fails and names the missing data rather than inventing a pace: a 6-minute test or a heart rate target solves it. Once you have run the session, the assistant can read the activity in Garmin Connect and compare each surge with its target.
Frequently asked questions
What is the difference between fartlek and intervals?
In fartlek, the recovery is always a run and the session can be free-form; classic intervals set precise distances or durations, often on a track, with a recovery that may be walked or standing. The 30/30 sits on the border between the two.
Can you run fartlek without knowing your MAS?
Yes. Free fartlek is run entirely by feel, and a structured fartlek can target a heart rate instead of a pace. For precise pace targets, the 6-minute MAS test gives you a baseline in one session.
Where should you run fartlek?
Anywhere: a path, a park, a road or a hilly trail. The free format makes the most of the terrain, and time-based structured formats need no track, since the Garmin watch counts the seconds for you.
Let your assistant write your sessions
Running Workout connects Claude, ChatGPT or Gemini to your Garmin account through the official API. Nothing to install, and no Garmin password to hand over.
See the connection guide