If you look at Clyde Hart's training theories (
http://www.nacactfca.org/articles/Hart-eng.htm
), he has his guys running up to 45 minutes at a time early in the pre-season, as well as workouts such as a 30 minute fartlek or 6 x 800 meters on a cross country or golf course. He acknowledges this isn't done all season long, but it is used to establish a base "in order that they can improve their oxygen uptake so that their recovery time between efforts will be cut to a minimum." I'd say Coach Hart knows what he is talking about, having coached Michael Johnson and Jeremy Wariner both to Olympic gold in the 400.
Read this quote from Hart:
"Generally the outstanding 400 meter runner will have approximately a one second deferential between their best open 200 meters and the time it takes them to run the first 200 meters of the 400 meter dash. The less experienced 400 meter runner should have approximately a two‑second deferential. A good formula for predicting the potential 400 meter time for 200 meter runners, providing they are willing to train and to give all they can to become a top 400 meter runner, would be to double the time of their best open 200 meters then add 3.5 seconds to this. It is obvious that the sprinter type has the advantage through the early stages of the 400 meters; however, if they are not trained properly, this advantage can melt away in a hurry toward the end of the race."
Basically, a sprinter type 400 guy who has a decent endurance base built up, but who has the same basic raw speed of the guy he is running against who has very little aerobic endurance background will be able to maintain his form better through the final 100-200 meters of the race.
I hope it didn't sound like I was advocating for a quarter miler to train like a 10K guy, but runs of up to 30, 45, maybe even 60 minutes once or twice a week VERY early on in the season will help a quarter miler with his hard workouts down the road, as well as his races.