There is no doubt about it, "fartlek" is a funny word. Say it out loud and it will make you giggle.
Knowing that the word is Swedish, might take some of the humor out of it, but no matter its origin it is a fun way to train. "Fartlek" is Swedish for "speed play" and is a variation on interval training. It consists of adding bursts of speed in the middle of a training run.
A fartlek running workout goes like this - Warm up running for at least 5 minutes, continue running at an easy training pace, add bursts of speed for various distances and times throughout the run. Basically you are mixing aerobic work with anaerobic work.
It is important to vary the speed of your bursts as well the duration of the speed sections. The speed section can last anywhere from as short as 15 seconds to as long as two or three minutes. One element of the speed section could be interspersing easy running with three or four "quick steps" now and then to simulating suddenly speeding up to avoid being overtaken by another runner.
The recovery pace is generally faster than the recovery jog you might do during intervals on the track; keep it moving at an easy training pace. The recovery time is usually about 2/3 the time of the speed section.
So, do you fartlek?