High mileage was commonplace in the 1970's. It was typical for every high school coach to recommend a 1000 miles over the summer for all cross runners after their freshman year. In three months do the math. Its high mileage.
Not surprising that distance school distance running was at its best the years 1974 -1978 in this country. And the all time peak for US distance running was in 1982 until the last 8 years which has greatly surpassed that. But there was about a 20 year period of slow decline which really hit bottom in 1996 when 28:10 10,000 runners were Olympians.
The observation was the majority of the best school distance runners didn't go on to become great college runners and beyond.
Couple that with the success of Sebastian Coe in the 1980's and Peter Coe his coach who popularized quality as the key and dismissed slow overdistance training as having no benefit.
And also the success of a freak talent like Bob Kennedy who reportedly never ran over 40 miles a week in high school and 60 miles a week in college. When you are the best everybody else figures you have it figured out and use your training as the example.
The Coe and Kennedy examples dually drove running in this country into the dark period of the 1990's. It was assumed that high school runners running over 60 miles a week would burn out.
A lot of credit should go to that high school coach of Dathan Ritzenhien who did moderate high mileage in high school and had success. In those days he probably got a lot of criticism for that.
The key is gradual development. If you have a real talent you want to leave some room for improvement and mileage is not the only answer but 70-80 miles a week for a high school senior that built up to easily handle that is better than keeping at 50 miles a week or the same running 100 miles a week at a young age.
There are now 20-25 US high school runners running under 9:00 now. Not all have the talent to keep improving to the top national class level but that movement brought back the 8:40 two miles and 4:01 miles that disappeared at the high school level for about 15 years.