jono wrote:
If it's timed, it's not a fartlek.
The way to do fartlek properly is to run fast no set distance or time, stop before you get tired and repeat and keep repeating until you are tired.
OK maybe you're technically correct, but fartlek has become more than the random speed play it originally was. The best fartleks serve a specific purpose. They are normally something like hard bits at anaerobic threshold pace, and easier bits at long run speed, but this can vary.
You can do 2/1s (like the Kenyans, and pre-programmed fartleks seem to work for them), 3/2s, or the Mona fartlek "invented" by Aussie marathon runner Steve Monaghetti. That one's 20 minutes total, with 2 x 90" hard 90" easy, 4 x 60" hard 60" easy, 4 x 30" hard 30" easy, 4 x 15" hard 15" easy.
But it all depends on how hard you want the hard bits to be, and how hard the easier bits are. Run the hard bits at mile/2km speed, and it becomes more like a vVo2max session, such as 7 x 2' at 2km speed, 2' at half that speed.
You just have to work out what you want to get out of your fartlek. Find a mile or 1km loop in a park, and run the better parts of the course (better surface, better wind) harder, and recover on the harder bits.