Been giving this a lot of thought as I am formerly a post-collegiate runner who transitioned into high school coaching.
It seems that the style of training that I was most successful with as a post-collegiate runner (and that which you often read about on the boards or watch on flotrack etc.) is not the best training for younger ages.
I believe that the younger the athlete the more they need smaller workouts both in intensity and in volume, but they need to workout more often.
Why I think it works
1. Younger people recover quickly
2. Less experienced runners are not capable of pushing themselves at a high level for as long
3. They are still developing physically
4. Neuromuscularly they can only handle so much per day.
5. (This is the big one). They learn to run HARDER and not EASIER.
5b. They train poor running form because of too much stress. In order to hit a difficult workout they compensate running form. Since they haven't spent years running they adapt running form and form bad motor patterns.
I have my high school athletes have a different stress every day of the week except Sunday.
I believe that as the athlete gets older they should gradually increase both the stress and duration and variation within each workout. This should continue to the point where they are able to put in one to two really quality workouts per week (usually of a mixed variety - which you see a lot of on Flotrack these days).
I know Ryun and others did succeed with extreme workouts in high school, but it seems to me smaller workouts every day is a much more effective method. Some elite runners use this approach as well (see Caleb Ndiku training), but I think it works better on younger athletes.
Lastly, I think this strategy mentioned above works best when the athletes are simultaneously really stressing improving their total volume of running. So while no workout is super tough the total load of the workouts is a lot for their age.
What do you think?