You're comparing apples to oranges. Riding multi-day bicycle race is completely different from the running a single marathon event. Some people run marathons every day, for a year (http://edition.cnn.com/2011/SPORT/02/05/marathon.record.enge...). It's hard to argue that that would require no less endurance than Tour de France (or Vuelta).
Yes, apples to oranges, but if we use total energy requirement and duration of a competition as indicator, than a typical road cycling race requires much more endurance than a marathon.
This is a classic one-day race, > 5000 kcal, > 6 hours.