Fartlek

Fartlek is Swedish for speed play: continuous running with unstructured changes of pace, speeding up and slowing down by feel rather than to a stopwatch. It sits between steady running and formal interval training.

The method came out of Swedish distance running in the 1930s as a deliberate reaction against rigid track sessions, and the informality is the point rather than a limitation.

Its advantage is variety without administration. You can run harder to a landmark, ease off, pick it up again on a hill, and finish having accumulated a genuine mix of intensities without measuring anything. For people who find structured intervals joyless, that difference decides whether the session happens.

It suits terrain that makes structured work impossible: trails, hills, unfamiliar routes, and busy paths where holding a precise pace is not realistic.

Its weakness is the same as its strength. Without structure, most people either drift into a moderate rut or produce a session that is unrepeatable, and progress is hard to track when nothing was measured. It is a poor tool for a training block that needs specific adaptations.

A reasonable compromise is loose structure: harder efforts of a rough duration with recoveries of a rough duration, judged by effort rather than pace.

In practice, use fartlek to add intensity variety and enjoyment to easy weeks or off road runs, and use structured intervals when you are targeting a specific adaptation and need to know whether it is working.