Background: I am 44, have run at least 40 miles a week for the last 30 years, at least 60 most weeks for the last 20, average over the last 20 years probably around 75-80 with sustained peak of about 100-120. Speed 2-3 days week. Still maintaining around 80. The most important thing that I learned about mileage is that there are other variables involved. If you get the other variables right, the mileage is beneficial for your health and fitness. If you get them wrong, you will be less fit and healthy. Fit and healthy go together - it is hard to make your body perform at top level if some parts of out are out of whack.
The specifics of the variables will vary from person to person as was already observed by several posters. For me the following was important:
- Adequate sleep
- Reduction of stress in life (managed to do this somehow while providing for a wife and starting with one and now up to ten children)
- Healthy diet - fruits, vegetables, grains, fish, light meat, no fast food or any food of poor nutritional reputation
- Avoiding harmful substances - in particular alcohol and caffeine - I realize that some people will debate whether those are harmful, but I consider those harmful for various reasons and stay away from them as a matter of principle.
- Split the runs into doubles, and even triples if the circumstances and creativity permit - which meant sometimes putting two babies in the stroller, and taking two running kids running to a grocery store.
- Understand what subsystem is the weakest link in the chain and be gentle to it. For me it is the adrenals. I noticed mine wear out at 6:40 pace or faster (for reference, my marathon PRs are 2:23:57a St. George, 2:27:46a Top of Utah, 2:30:32 St. Jude), and kept my easy runs slower than that. If I got bored, I would just run one quick mile to "get the devil out of my legs", and that was usually enough to keep me happy for the rest of the run at 7:10-7:20 or even slower.
- Stay out of trouble. I noticed that if I ran more than 5K on the track on a frequent basis or on trained on slanted surfaces, it would irritate my lower back, but I could run on asphalt without any problems, so I trained on asphalt using the track only for occasional fitness tests/sense of pace calibration.