Which segues into the real point of this post: the hardness of the two main types of road surfaces. I think conventional wisdom is mostly all wet when it comes to the hardness of running surfaces. Sure, as I just pointed out, running on dirt or pine needles is clearly softer than running on a road. But beyond that I disagree with the conventional wisdom, which is that macadam (blacktop) is easier on the body, impact-wise, than concrete.
Sure, you could drive a nail more easily into macadam (just ask the carnival guys who pound tent stakes into macadam parking lots), but I submit that there is no appreciable difference when it comes to running. For there to be a real difference in impact, when your foot strike happens, the macadam would have to give in a measurable way. Your running shoe would actually need to leave a discernible footprint...and of course, it doesn't.
My opinion: Think placebo effect. Macadam may seem softer, and mentally we want to think that if we can't run on trails, macadam running is the next better choice, but it really isn't any softer than concrete. Nor would I think that running on a boardwalk would be any better than macadam or concrete, either.
Of course, this would be a perfect doctoral thesis for some aspiring physics major. And I'd be willing to be proven wrong.