I never succumbed to the lures of high mileage. I was a member of two intercollegiate championship cross-country teams, yet I don't recall doing more than 5 miles in any given practice. When I returned to running, I found that the veterans I met trained 5 miles a day and ran a race every week. And that's been my practice ever since. Except for races, I remember running more than 10 miles only once or twice.
We forget that Roger Bannister broke the 4-minute-mile barrier on 5 hours of training a week. Bannister later wrote that an additional 10 seconds of improvement would require four times as much training.
Research by exercise physiologist Dr. David Costill confirms Bannister's theory. Dr. Costill found that the greatest percentage of improvement occurs with 25 miles a week. Close to maximum improvement takes place with 50 miles a week. Beyond this––training as much as 200 to 225 miles per week––resulted in no further increase in maximum oxygen uptake or the ability of the heart to pump blood. In fact, training beyond 50 miles a week reduces a runner's anaerobic strength, which affects speed and the "push" at the finish.
I discovered these truths on my own. First I found I could run a respectable marathon on 30 miles of training per week. By observing my colleagues, I also figured out that if I doubled my weekly mileage, I could probably improve my marathon time by 5 minutes. But in return, I would risk the staleness, illness, and injury that comes with excessive mileage.
George Sheehan, 4:47 mile @ Age 50, 3:01 marathon @ Age 61