Currently, I'm running over 35 miles per week. By mid-June, my mileage will be close to 45 miles per week. In comparison, while training for the New Orleans marathon in 2004-05, I ran less than 30 miles per week. So I've ramped up my mileage significantly. I've also added some quality runs that are much more challenging. Track, hills, and tempo workouts really boost your threshold and tolerance of running faster & harder.
However, some intense marathon training programs ask people to run 55 to 75 miles per week. That's a whole lot of miles!! I could imagine running closer to 50 miles per week- probably by extending one weekday run to around 12 miles and running about 20 miles on the weekend. But how do you keep your hips, knees, calves, tendons, and feet happy when you're subjecting them to THAT many miles?? I'm not certain that I'm built to run 55 or more miles per week- plus that would take a lot of time :O I think it's worthwhile to see the fruits of this marathon training cycle before checking out the extreme-high mileage plans.