I have done both and can say without hesitation that a Century is WAY easier than a marathon.

In fact, I ride one or two centuries every weekend. I could never run more than one marathon in a weekend, nor would I even have interest in suffering through even one a month . Heck, maybe one a year.

Marathon distance is just brutal on the body... all of that pounding and jarring of your joints, your intestines.... I can't even explain it - one has to experience the joy for themselves.

I'll stick to cycling from now on, thank you.