I don't really buy that cycling is "gentle." It's true that it's not high-impact, but that's mostly about joints (IMO, I am not a sports scientist).

And joint stress is probably the reason that most people could not run a marathon every day for 3 weeks. The muscle energy expenditure for the TDF is off the charts--typically 8,000 calories a day, and the vast majority of that is burned in the leg muscles and respiratory system.