I'm trying to improve my times in the half marathon and the marathon, so I've been reading a lot of training plans for races at those distances. One thing I've noticed is that most of these programs tell you to do your long training runs quite slowly, often recommending 60-90 seconds per mile slower than your target race pace. Why is that? That is, it seems like if I want to become a faster runner at a certain distance, the best way to train would be to run that distance, quickly. Yet most plans instruct you to do your long runs slowly, and to build speed in separate, shorter workouts. I can see the benefit of doing shorter workouts at a faster pace, but I don't see how doing my long runs at 60 seconds/mile slower than my goal pace is going to help me as much as doing them at as close to my goal pace as I can handle. Isn't that how most training works?
So why is that? Is it just to keep my legs fresh for other workouts? (If so, then couldn't I run faster if I know I'm going to have a day or two of rest following my long run due to my schedule?) And is there empirical evidence to support that these long, slow runs train athletes better than faster runs of the same distance? Or is it just unquestioned coaching orthodoxy?
Edit: There are some good answers below, but I haven't accepted any of them because thus far no one has been able to point to any scientific studies. They may not exist; they often don't for this type of thing (it's almost baffling how little hard, scientific evidence we have for so many of our most deeply held beliefs regarding fitness and exercise). That's what I'm hoping to find, at any rate.