I think it has more to do with them having to go hard at the gym later. Gym work would be a lot harder coming fresh off of a 5+ mile run and the straight into the gym. If you get your run in early you can have some time for your legs to rest then get into the gym for another workout. I know I wouldnt be able to go through a couple of hours of doing bag work, focus mits, sparring, skipping rope, etc.... if I just got done running.