Fartlek, fartlek, fartlek. It is so fun to say and doing them makes your training more fun and more effective. Here's the not so technical way to describe what a fartlek is . . . adding speed bursts into to your training session. The distances you add speed and the speed are up to you. When I do them I warm up for about 1/2 mile, then sprint for about 1/4 mile and then I slow down to a steady speed for the same distance as the fast run and alternate between street signs in the sub - fast, comfortable, fast, comfortable to complete 5k.