Push the Boundaries Out a Little at a Time (by John Kellogg)
During the off-seasons, experiment with going slightly higher than you've ever been for a few days in a row. It doesn't have to be done in 7-day blocks; 3-5 days at a time of higher mileage is fine at first.
Recover from that injection of higher mileage with a lower week or two, then go up in mileage for several days again.
Run EXTREMELY RELAXED during the times which you are pushing up your mileage to previously un-attained levels. DO NOT increase both mileage and intensity at the same time until you've done several seasons of each SEPARATELY. You CAN run some short (15-30 secs.) buildups or strides every other day to provide variety, but avoid "knee-grabber" anaerobic workouts and races.
With each passing year, you'll be able to increase both the mileage itself and the length of time you're spending in your high mileage phases. By the time you're in your mid-20s, you should be capable of several WEEKS in a row of VERY high mileage without needing a low week and without having any stress-related problems.
Your average pace will probably also become much faster than it once was, but you don't need to FORCE that. At some point, you'll find that you can hit a pace at near the high end of aerobic effort on MANY (if not most) of your easy runs and will recover within a day's time. You must always start laughably SLOWLY on EVERY run in order to do this type of training successfully, though, and it's not something you should consciously worry about improving. Just allow the faster running to come to you over the years as your body becomes ready for it.
As far as mileage goes, though, start INTRODUCING it in short stints when you're YOUNG, but allow
your body to "absorb the training" by dropping back down in mileage for a little while between high
segments. Push those boundaries out more and more (and with ever longer stints) from year to year as you mature.