Hey Man,
I think mileage is a poor measure of output. It's something that's pretty damn conventional among runners but it's basically useless.
For example: 100 miles at 7:30 pace is a hell of a lot different than 100 miles at 6:30 pace. For starters the intensity is different and so is the duration. Running faster kicks your heartrate into a higher zone for a shorter period of time which definitely has its benefits, but running slower at a lower heart rate for a longer duration also does good stuff for the body.
I had a great coach in hs who taught us this and recommended that instead of counting mileage, we wear heart rate monitors and record duration in a set zone. We used three zones. 1 really light running warming up jogging stuff up to about 140 beats per minute. 2 moderate stuff distance run up to 165/170. And then harder above 165ish.
It works even better for cross training. For example, XC skiiers only record time and intensity, not mileage or anytyhing else. And they train hard. They often exceed 30 hours weekly.
I would say if you're serious about getting fit, and are 17 years old or so, a good workout would be 30 minutes in zone 1, 1 hour in zone 2, 12 minutes or so in zone 3, then another 30 minutes in zone 2, followed by 10 or 15 in zone 1. Total workout time approximatley 2:30 or 2:45 minutes.
If you supplement that with light running to keep your muscular mechanics, you'll be pretty damn good by the time you can run a normal amount again.