Originally Posted by
agarose2000
Cycling and running can both be hard - but make no mistake, the pounding on joints on running definitely makes it "harder" to cross over from cycling->running versus the reverse.
Marathons are WAY harder than century mile bike rides. It's not even close, in my book, and I've done both. That said, you can definitely bike for much longer than you can run during training - 6 hr bike rides are longish but routine for serious cyclists, whereas 3 hr+ runs are simply too long for runners for training, even at the elite level. (They do multiple shorter runs during a day to get 4-6 hrs total.)
It all depends on how you do each. Running a comparably fast marathon (under 3 hours) is about the same as a solo century under 5 hours riding time. Both take some hard training and both will leave you exhausted at the end. The perception that marathons are harder is running is a weight bearing activity so your body feels beat up.
One can "do a marathon" by walking and slow running. Someone can also "do a century" by slow riding and resting at stops. So the comparisons need to be on a common point of reference