Maybe that would be a sliding scale, on what distance running is equivalent to a century. I think that at our age the low impact training for a century is a lot easier than the high impact training for a marathon, while for someone in their 20's the stress of impact may be no priority. Since the stress and impact is what I hear most often from runners who decide to take up biking, and even those who still run but only at a reduced pace.

FWIW 10 or 11 mile run wipes me out, about the same as 65-80 miles on the bike. So I'd guess that a half-marathon would be closer to a century than would be a full marathon.