Actually, amithereyet (great name, btw), the C&O Canal towpath is entirely flat. There are a few short rises at some of the locks, but the total elevation over 184.5 miles is less than 200 feet. It's a tad rugged, what with rocks, gravel, tree roots, and holes, but other than that a very easy, enjoyable ride. This site's a great resource for more info.

Oh, and I have the very cool "On the Canal" dvd if anyone's interested in borrowing it before the big trip.