High mileage, through chronic depletion, improves the ability of the muscles to utilize glycogen (more can be stored).
Long runs improve the "plumbing" (particularly venous return) and also cause the body to produce more energy from fat.
Since you can only obtain the ability through training to store enough glycogen for 90-120 minutes, it really comes down to how fast you can run a marathon. If you can be fast enough to run 2:30-3:00, the amount of energy from glycogen is large compared the about you need from fat, so the higher mileage runner with larger glycogen stores can get by with less fat utilization, and thus less long run training. But in a 4:00 runner, you need much more contribution from fat, and you won't see the same effect (unless much higher mileage makes that person a much faster runner), which is why the RW-type prgrams place considerably more overall emphasis on getting the long runs in.
Keep in mind that for a truly competitive runner going for high placing and maximum performance, it is not about what you can "get by with" but how much training you can do without overtraining, and how you distribute the training loads for greatest results. Such people do get benefits from long runs, but those long runs are often shorter in terms of time (but not necessarily distance) than for a slower runner that gets proportionately more benefit from fat utilization.