Plan a fartlek running session.
Fartlek — speed play — alternates surges with easy floats without stopping, so the main set is simply as many surge-plus-float cycles as fit in the time remaining after the warm-up and cool-down. The hard share is the number worth watching: a fartlek session typically puts 15-25% of total time at effort, which is what distinguishes it from a track interval session where recovery is walked rather than jogged.
Fartlek structure
Main set seconds = (total - warm up and cool down) x 60; surges = floor(main set / (surge + float)); hard time = surges x surge / 60
These figures are population-average estimates for training and recreational planning only and are not medical advice. Individual responses vary widely. Consult a qualified health professional before starting or significantly changing an exercise programme, particularly if you have any medical condition. Official dimensions, rules and timings should be confirmed against the governing body for your sport.
Continuous running with unstructured or loosely structured surges of faster effort. The float between surges is easy running rather than standing rest, which keeps the aerobic stimulus going.
Around 5 km to 10 km race effort for longer surges, faster for surges under a minute. It should feel controlled — fartlek is not a series of sprints.
Intervals use measured distances and full recoveries on a track; fartlek uses time and jogged recoveries on any terrain, which makes it more adaptable and less precise.