Walkin' vs Runnin': a Case of Smooth vs Harsh
One of many interesting ideas from Ray Jardine: it's fairly reasonable, even pleasant, to walk 25-40 miles a day for as long as you like, weeks on end. But running anything like that for more than a day just shatters you. The (trained) bod evolved for walkin' -- if it's distance you want. After a marathon+ run you usually need days/weeks to recover. Check out his "Trail Life" (newer; "Beyond Backpacking" is his classic). The Paleo tribe says running is only evolutionarily ergonomic in the short-range.
Ray is known as the godfather of ultralight hiking and fast distance hiking. Bikers have taken it up, too.
Here's a link to Ray's books. (And OYB gets a cut of anything you buy on the visit!)