Originally Posted by PatW

Running is pretty high impact. The high impact often produces injuries in the knees and feet. I have the notion that running "beats up" the runner. A marathon really wrings out a runner. Also you don't hear of runners running 2 marathons on days back to back.

Cycling is low impact. As a result, cycling allows the person to sustain the exercise far longer than a runner can. I figure a century (100 miles) is about 50% more effort than a marathon, but subjectively it does not "feel" like it. Sure a century is a decent ways, but I have done centuries on four consequtive days. I could have done more but things like life interfered with my cycling. I have never heard of a runner doing 4 marathons in a row (but I suppose someone has).