I wouldn't worry too much about endurance. I have done the entire C&O canal a couple of times and it is not a physically demanding ride. The toughest hills you will encounter will be coming off the C&O to get to Antietam. I rode that route last summer with my 10-year-ol son and he had no real problem with the hills. Whe we road the battlefield one of the rangers was nice enough to mention that the southern end (Down by Burnside bridge) was pretty hilly. We decided not to ride that part of the battlefield and instead stayed on Harper's Ferry road through Sharpsburg. That proved to be a wise choice for us and made for a very nice ride.

Other than the section from Great Falls to DC you will be amazed at how empty the towpath is on a typical day. From Great Falls into DC let me apologize in advance for some of the dumbests most incosiderate tourists and trail users you will ever find. The widewaters area just below great falls makes up for an awful lot of stupid trail users, but be cautious. Good luck with the ride, its a trip I'd love to find time to ride next year.