I don't understand wrote:
So....if someone is a 400/800m-type and by definition weaker aerobically (than a 800/1500m-type), then wouldn't they reap benefits from increasing their aerobic capacity? Why not run more miles in the fall? Why not do a couple of longer aerobic capacity workouts every week?
Hang on. I never said don't work on improving your aerobic capacity. I said it shouldn't be all you do - long steady mileage I mean.
An 800 runner in base - IMO should be doing speed development, strength work to be able to run flat out, and higher intensity aerobic work, like structured fartlek, vVo2max (same thing sort of). If they've had a break I can see the benefit of starting with some longer steady stuff, but only for a month or so till they start getting aerobically fitter. Start long and short, and work your way into the middle.
For example I don't see what longer steady running (say 50-60 mins) does, except make the athlete better equipped to handle 30 mins of intense fartlek, which CAN directly help an 800. The good athletes are only running for 100-120 seconds. They need to be able to handle a fast pace for the first 500 and be able to bring home the last 300.
In a sit kick race, speed is more important. In a fast time trial type race, aerobic endurance becomes more important. In both cases of course, you need both.
Even Rudisha is vulnerable, because he's beatable in a slow sit kick race. He has good 400 speed, but others have better top speed (Aman, Amos). He knows this, and it's why he pretty much runs all his races one way. Fortunately when you go 49 for the first lap, you control things.
So I don't think being able to handle a hard 25 mins tempo at close to 10km speed is that important for an 800 runner. I think you need to do the sort of aerobic work that benefits the 800. So 5 x 3 minutes at 2-2.5km pace with 2-3 mins jog recovery is of more benefit than a tempo, which might help a 5km specialist.