Why: Fartleks prepare you for longer, more structured workouts down the road because they combine short bursts of harder efforts with short rest periods. You're the boss when it comes to fartleks; you decide how long (or short) each interval should be.
Who: From Jay Johnson, former coach of national champions
If you're indoors, you can simply get down to your threshold pace and run it for a few minutes, then back off to a pace that is faster than your easy pace, but still slow enough that you recover a bit. My college track coach called this pace "steady." Then you repeat it--faster running followed by steady running. I like to just go by feel and say, "For 50 minutes I'm just going to play with pace." Fartlek is a Swedish term that means "speed-play," and I assign it for athletes because I want them to learn where their aerobic threshold is by playing with pace.
But if you're someone who needs more structure, you can do a workout where you run five-minute blocks of time. You can run two minutes at your threshold pace, then back off for three minutes, then repeat. We call that "two minutes on, three minutes steady."
Complete a short warm-up and cooldown that consists of 10 minutes each of easy jogging.