Besides being a word that makes my children giggle, “Fartlek” is a Swedish word that means ‘speed play.’ In training, it is simply playing around with speeds, sort of an unstructured form of speed work. It involves a continuous run in which periods of faster running are mixed with periods of easy- or moderate-paced running (not complete rest, as with interval training).
I have fun with this. I will pick a distance or an amount of time to run at a particular pace. Sometimes I will combine this with a challenging terrain. Afterwards, I return to my original pace. When I run fartleks, I usually plan at least 3 into the run depending on overall distance.