For me a century is a piece of cake. I could not even imagine running a marathon. The only way for me to equate a century to a marathon would be if I were allowed to walk the marathon. In that case, they would probably be similar and take about the same amount of time to complete.
I think I could walk a marathon at about a 3.5 mph pace and finish in about 7.5 hours. I could probably do a comfortable century at about 13.3 mph and also finish in about 7.5 hours. I'm sure that my body would feel the effects of the marathon much more than the cycling century. Walking/Running are weight bearing exercises and an individual would expend more energy with this type of exercise. I imagine that a cycling century would include segments that would include "coasting." That's basically taking a break. Running or walking... if you want to move forward, you have to propel yourself.