It is very likely that the often beaten-to-death topic "Oh, I wonder whats more difficult, running a marathon or riding a century" pops up in our minds. Its a good question nevertheless, one that makes for a good round-the-table or commuting-to-a-race conversation.
A little Google search for a calorie counter could get you the most quoted answer - a marathon is more difficult to do.
As a bike rider, I'll probably tell you the same thing.
But why is running more difficult?
Before you make this a black and white paradigm, there are many things to consider.
I've outlined them in this little chart below. Note that for every factor considered, I've attached a "Pain Factor", a weightage number anywhere from 0-10. Ofcourse, I could be biased in assigning numbers but I hope you see the big picture and not just concentrate on figures.
Let me know if any of the information is wrong, or if I've missed out additional factors to consider. Also, your perspectives in riding and running are also welcome.
UPDATE : JAN 3, 2009
There is couple more important things I seemed to have missed. This may require the pain factor to be adjusted, but I'm not going to do it. Just consider these two points :
1) Cycling is definitely more forgiving to age and weight than marathon running. Sure, the folks with good power to weight ratios get uphills faster, but heavier riders go downhill quicker. And thinner riders may not be the best time trialists, but heavier, more muscular riders can pedal steady bigger gears for longer periods of time. Running also gets difficult with age. As you age, you lose bone mass, your muscles atrophy, and your achilles tendon or shin will at some point say no to the things you could easily do when you were in your 20's. However, cycling features pushing a series of gears so you can always make your task easier by selecting the right gear.
2) Center of Gravity : Running, especially a marathon, is definitely impact intensive. It also involves the bobbing up and down of the body, or more specifically, the center of mass. To raise your mass and lower it, and raise it again all the while stabilizing your body weight on two feet for many many cycles definitely requires energy. In cycling, the center of gravity is more stable. Pedals, saddles and handlebars act as support points for your body weight. A lot of your work in trying to remain upright is taken care of by the bike itself.