Don't underestimate the difficulty of the C&O part of the trip. Yes, it is all flat, so it's easy in that regard. But the surface can vary from reasonably good to horrible depending on the section and the weather. If it rains, lots of mudholes. It's basically a double-track path for 180 miles. I did Cumberland-DC in three days @ ~60, 60 and 70 miles and it wasn't too bad (Cumberland-Hancock, Hancock-Harper's Ferry with Antietam detour, Harper's Ferry-DC), but I had good weather. Nevertheless, the constant rough track really took a toll toward the end of each day with sore butt and hands. On several occasions I detoured out onto nearby roads just to get a break from the path and the sometimes monotonous scenery: I went in August and it was mostly a green tunnel the whole way. Pretty, but it got old after a while. After I finished I didn't think I'd ever really want to do it again, but now that some time has passed I'm thinking of it again. But not in the summer. Too many bugs, too hot, and not enough scenery. I'd like to do it in the spring or fall.
A particularly nice deviation was from Williamsport to Antietam and then through Sharpsburg. Some really pretty, small, rolling roads past farms and a small village or two. You'd have to do a detour anyway in that section due to the closed section of the path, so might as well see some of the countryside. Definitely visit the Antietam visitor center and some of the battlefield if you have time. Nearby Sharpsburg and Shepherdstown are nice, too (more in Shepherdstown than Sharpsburg).
I camped/stayed at the hostel in Harper's Ferry, but for motels/B&Bs I'd definitely make reservations, as there aren't very many lodging choices along the route. The Harper's Ferry hostel is actually pretty nice, but it's at the top of a big hill and there is not much around it (no restaurants, food stores, etc.).
For the C&O I ran 1.25 road tires on a MTB that I built up as a touring bike. Handled the path fine, even with a full camping load. I understand the GAP is a much better surface. I hope to do the GAP this year to finish out the full route. Logistics can be kind of a pain, though, getting there and home.