fartlek

It's fun to say and it's fun to do. Swedish for "speed play", fartlek training is a type of interval training used to increase athletic performance. Unlike traditional interval training, where measured bursts of speed are followed by roughly equal recovery periods, a fartlek session allows the athlete to decide spontaneously when to surge and when to taper. Often the most productive fartleks are in hilly terrain (running and cycling) where one goes uphill at full exertion and then coasts downhill.

I find this type of training to be much more enjoyable than traditional interval training because you spend the time listening to your body instead of being a slave to your stopwatch.