Having done several centuries and not much running
I feel very comfortable saying that a century is much easier than a marathon. I'd say that with about the same level of training for both sports, a half marathon on a hot day is a little bit harder than a century on a cold day.
Even if you're trying to do a sub-5-hour century, the bottom line is that cycling is low-impact and running is high-impact. No amount of training will even these two aspects out.
Or, think of it this way. Let's say I spend 3 hours on the bicycle, riding in the aerobic zone (70-80% of max HR), and the next day do a 3 hour run in the same HR zone. The run will still beat you up far more than the bike ride.