Perhaps I haven't been clear.
Let me ask you a question:
If you run 3,000mi in a year, are you going to run 60miles every single week?
No. There's base training (where volume would be in the 90 range), racing (where volume would be in the 40s), and periods in between (50-70mpw).
3,000mi/yr and 90mpw for certain parts of the year are not contradictions.
For example
Fall Base (Avg. 80mpw)
Winter (indoor) (Avg. 55 mpw)
Spring Base (Avg. 90mpw)
Spring-Summer Pre Track (Avg. 60mpw)
Summer Track (Avg. 35mpw)
That's an annual average of 65mpw or 3250mi/yr.
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And with regards to singles, I think that running only in singles inhibits your recovery. 2x 45 min (am/pm) runs will function as recovery while 1x 90min run will not. I think it's easier for guys to get hurt running that kind of volume all in singles.
I know what Wetmore's after (more capilarization, endurance....) but I think the risks to injury outweight the benefits, which are probably miniscule. Lydiard's guys, Geb, the whole lot ran/run twice per day.