Adaptation is underrated. Just as a runner who has never run more than 4 miles in his life will probably be in a boatload of trouble trying to run 20 miles; but can work up to it over time, a runner who hasn't done much/any running on concrete probably won't feel very good, and might even injure himself when he first tries it; but should be able to adapt to it over time. I don't need to be told how much harder concrete is than ashpalt or grass; I just don't think the difference is as important as an individual's capability to adapt to that difference. If this weren't the case, we'd be coating tracks in sh!t for all of the distance events, because anybody who has ever run through a pile of sh!t can tell you that its softer than the track is. I run less mileage than a lot of people here, and I'm also substantially slower (60 - 70 mpw on average, marathon barely sub 3); but I've been doing this mileage in racing flats on concrete for over a year, and my only recent injury was from tripping over a dog's leash and doing a face plant. If I, a 140 pound relatively slow guy, can run 70 mpw in Air Streak Ekidens without any problems, I can't imagine why on earth a fitter, stronger, lighter runner (like most of those here) would have any trouble doing as much as 25 - 50% of their running on concrete.