The "1000-mile" summer for motivated seniors, but no more. 70-80 mpw is a lot. I remember the best 10,000 meter runner of my day way back when in Todd Williams ran something like 40 mpw as a soph, 60 as a junior, and 80 as a senior. Some guys have success with 90 and above, but the risk/reward/burnout equation goes up significantly. Also, I wouldn't personally have them running doubles until they can handle 60 in singles, something like 8 every day and a 12-miler on Sun.