Fartlek

Fartlek is a run in which you vary the pace as you go, without fixed intervals or rest periods. You might pick up the pace to the next lamppost, ease off, surge up a hill, and settle again — the structure is loose and driven by feel or terrain.

It matters as the bridge between easy continuous running and formal intervals. It introduces higher intensities without the psychological weight or the precision of a track session, and it teaches pace changes in a way that transfers well to racing, where efforts are rarely evenly spaced.

In practice it suits base-building phases, runners returning from a break, and anyone whose interval sessions have become a chore. Because effort is self-selected it is also forgiving on days when you turn up flatter than expected.