Running, at least for me, was tougher. It also took longer to train for than any century I have done. My last hilly ride was Savage Century (12k of climbing) and it wasn't as hard as the marathon. When I was training for the marathon I essentially had to stop riding on Saturdays because the Sunday long runs got to be too hard. Generally I can ride both weekend days.

I still prefer riding, but I kinda do miss my morning runs in the fall around DC.