The simple concept with the long runs is to go slower. If you start slowly, use a 1-1 throughout, and don't pick up the pace coming back, you will recover faster and get all of the endurance of the long run. You cannot go too slow on the long one.
Don't pick up the pace on the return. This is OK in a race. But injuries in long distance running tend to come from just this type of behavior. It will not help your fitness to run faster in the second half of a long run--you will improve much more by doing the speed repetitions.