I was a sub-3 hour marathoner in my late 20s. That's nowhere near good, but it is far better than average. I would say that running a fast marathon---say under 2:40---is way tougher than riding a century in 5 hours.
When you talk about just running a marathon to finish--say in the 3:30 to 4:30 range, then it might be easier than riding a century averaging 20 mph.