I think they are "strides" Peter. Fartlek literally means "speed play" - it's not always short bursts of speed. So you may sprint flat out between 2 lampost distances 6 times, or run at 70% effort up a hill, or jog for 3 mins, or incoporate 2 miles at 5K pace followed by a mile at an easy pace - it just means "playing" with your pace. To me a fartlek session is the whole session - I'd warm up for half a mile, do, I don't know.... 5 miles for example of fartlek - incorporating different distances of speed, hills and at varying speeds then a half a mile cool down. That is my understanding....