joejoeyjoejoeshabadoo,
Right now i'm running over 22 mile runs 3 times a week. Thats after running about 80mins or longer in the morning, the long runs are around 6 min miles, currently i'm integrating some workouts into the long runs, but slowly. And no i'm not doing only 3 days of running a week, i'm running over 200miles, so no its not THAT hard, but difficult nonetheless, its a lot of pounding.
As for the issue of running singles or doubles, we can beat this one to death, the fact of the matter is, running more makes your body a better running machine.
Running a marathon is an aerobic event, so the better the aerobic stimulus the better you will be at marathoning, so for a given amount of mileage you want more longer runs.
If you are an 800m runner or miler like steve scott, the emphasis is on the hard workouts you do, aerobic development plays a smaller role. SO you do less long runs because they tire you out for the intense sessions that are so much more important. The focus is different so the way you run the mileage is different, simple as that. In one its a primary stimulus for break down, in the other its a primary stimulus for recovery.