"Mileage is over-rated. If you can run a crap-load of miles, good for you. If you can’t, you can still improve a hundred other ways."
From Lauren Fleshman's blog -
http://asklaurenfleshman.com/questions/2011/02/16/1788/
This is actually something I've thought for a long time. I also think it may also mean that if, for example, you are a guy and you can't run more than 60 miles a week without getting injured, then you may just not have the body to be a really good marathoner - maybe you should be running a shorter event or maybe you just don't have the body to be a top runner.