The trouble is that cycling and running are so different. In terms of absolute caloric expenditure, cycling is the "winner" hands down. Running is just so hard on the body that few people can run for the same amount of time that a decently trained cyclist can ride. IMO, the demands of the century in many ways exceed those of running a marathon. The average marathoner may discover that he/she is poorly prepared for the 5-7 (9? 11?) hours of exercise that a century requires.
Having said that, a century cyclist can complete the distance at a significantly lower average heart rate than can a marathoner. The bike allows you to take it easy, relatively speaking, where the runner rapidly discovers that below a certain heart rate, he is no longer technically running. So the "beginning" marathoner may well find that his fitness is superior to that of the "beginning" century rider.
Last edited by Six jours; 05-18-10 at 08:54 PM.