Your fartlek workouts should be completed in five-minute blocks. These are my two favorite fartlek workouts:
Three minutes on, two minutes steady. You can do this for as little as five sets, or 25 minutes, all the way up to 10 sets, or 50 minutes. Start with this workout because you get enough recovery in the two-minute steady segment to lower your heart rate and be ready for the next "on" portion. Complete a 5- to 10-minute warm-up and a 5- to 10-minute cool down before and after the fartlek.
The next workout is simply four minutes on and one minute steady. This workout is much more difficult and one that you'll tackle only after you've done several of the three on, two steady fartleks. Again, you can do as little as 25 minutes (five sets) for this workout or as much as 50 minutes (10 sets).
Progression runs are simply workouts where you run faster as the session progresses. The following progression runs start slower than threshold pace. You'll run at threshold pace at some point during the run, then at the end you'll run faster than threshold pace.
The key to these workouts is to be able to say: "I could have gone 5 to 10 more minutes at that last pace if I had to." If you can say that, congratulations; you ran the workout controlled. If you can't say that after the progression run, then you likely ran too hard. This is a problem because you don't want to have a workout in the middle of your training that is closer to a race stimulus than a workout stimulus. Again, running controlled is how you should train—you rarely want to run "all out" in workouts.
More: How to Run Relaxed
A simple progression run is 20 minutes steady, 10 minutes faster, 10 minutes faster. If you do a 10-minute warm-up and a 10-minute cool down, then you get a solid 60-minute workout. You have to change pace twice, which is more difficult than it seems. With that in mind, you should run the first 10 minutes conservatively to ensure that you will be able to speed up during the second two 10-minute segments. You know you've done the workout correctly if you can say that you could have done 5 to 10 more minutes at the final pace of this progression run.