I guess slow twitch is not the best way to say it, but what I mean is they have more muscle fibers that use oxygen to fuel them.Its not the technical term for them but they are known as slow twitch muscle fibers.Sprinters will have a lower number of these slow twitch fibers while distances runners have very high number of these fibers.These oxygen fuel muscle fibers can't create as much force but can be used for longer. Muscle fiber composition will greatly effect how you respond to training.
You will not lose your speed just from doing more mileage, but if 70 percent of your training is slow running you will not train your muscles to produce maximal force and you will decrease speed as a result. A 3:50 mile is fast for the mile but it is not run at fast pace a 1:41 800 requires around a 48 first lap. A 3:50 mile takes around 57 sec evenly paced Laps.Galen Rupp is not fast and his races don't require him to be, but in a 800 speed is a key limiting factor. Galen Rupp has to do very little to maintain his speed because there is simply not alot of it.Jacob is faster but doesn't have a lot of speed as his father said.With Hassan well the women's 800 is 6-9 sec slower per lap its a different event than the mens 800.
This is pure conjecture, but I think most 400/800 runners chose to be mediocre sprinters in college. Its hard to convince a 46 to 47 sec 400m runner to run the 800.
Sprinters have ran 1:47-1:49 with little to no 800 specific training, these are the 400/800 guys.Rudisha was a 400m runner who ran 1:49 on his first try on a dirt track.
I personally view the 800 as a hybrid event sprinters and distance runners can excel at it but they don't train the same way.A 400/800 may not need more than 30mpw for them 50mpw is excessive.