Mileage for Mileage sake actually may pay off in the aerobic arena (I'm a lydiard fan)
Maximize the time you have before doubles. So if you have an hour, run an hour.
There is a limit. If you unemployeed and have the ability to run 2 to 2 and half hours a day, i still wouldnt. Mark Wetmore (head coach of buffaloes, lydiard student) mixed the two theories of single training runs and high mileage, and i bealive this had alot to do with the injuries that came along in the book “running with the buffaloes” I personally do not condone running very long singles over and over. Even now doubles are more present in his training then before.
I think long singles have there place, but the stress they put on the body is comparable to a threshold run or intervals. (Intervals stress the energy systems/glycogen systems more, long runs stress the physical more)