Hello guys, thanks for giving this workout its own thread. I thought I'd jump back in and clarify some things from my original post:
I wasn't clear about which day to do these hill sprints. You should always try to do this on a WORKOUT day. The only purpose for easy days is recovery, so keep those days easy. In college, it was very common for my team to lift only after workout days, the same principal applied with the hill sprints.
I noticed that someone asked if this type of workout would be too much following a workout day. I guess that depends on the training phase and workout preceding the hill sprints. The conversation I originally posted to was about building speed during a base phase. So the workouts I'd expect to proceed this speed work could include long runs, fartlek work, progression runs, steady states/tempos, and even longer or high volume intervals (mile repeats or 20x400m for example). You should absolutely do hill sprints after these types of workouts, for a couple reasons:
1) Endurance work has a tendency to cause athletes' form to breakdown towards the end of the training session (picture a runner progressing his pace over an 18 mile run, those last 2 miles, especially during a base phase, can look ugly). Hill sprints force an athlete to maintain form (sprinting in general causes your biomechanics to tighten up, but adding the hill helps force athletes up on their toes, mandates knee drive and arm swing). It's good to "remind" the body, especially when tired, the way it should be moving.
2) From a physiological perspective, sprinting after running "slow" (respectively) makes a great deal of sense. Aerobic intensive work, such as long runs, progressions and tempos, usually recruits slow twitch muscle fibers. By the end of a 10 mile tempo, for example, these slow twitch fibers are totally exhausted. Now if that same runner who just completed a 10 mile tempo, and is exhausted, now tries 10-12 100 meter hill sprints, that runner's body will now recruit everything that isn't exhausted. For distance runners, this usually means that their fast twitch fibers are heavily recruited, for these fibers are rarely utilized during these "base phase" workouts (by their very nature, distance runners are slow twitch athletes). By doing hill sprints after these workouts, you effectively isolate fast twitch fibers and can build them up more effectively. Imagine trying to build up your bicep muscle, wouldn't it make sense to do a lift that isolates the muscle? That is why hill sprints following a workout is the messy effective way to build up a distance runners top-end speed.
Last point, regarding recovery. I know it may sound crazy to do roughly 2000 meters of sprints AFTER a 20 mile long run, or 20 x 400 meters. But keep in mind that these hill sprints must be approached the same way you do mileage, gradually. If you allow an athlete to build up this workout the same way you build up his weekly mileage, they won't even notice the effort required to do 2000 meters of speed work after an aerobic intense training session. That's all mental, and it takes a good coach to motivate athletes beyond the mental barriers. As for recovery, this workout will not break you down (although early on you can expect soreness). The aerobic work is more likely to cause cumulative fatigue, but that is why you need to be diligent about recovery days.
Hope that helps! (FULL Disclosure: I am Andy Arnold, the guy who was in Kenya this past summer, working for Nat Geo and writing occasionally for letsrun.com.)