fallstoclimb, yes, use a hybrid on the C&O. I did the C&O a couple of years ago; here are two threads I started, the first about the prep (lots of good suggestions from folks here, which I really really appreciated!!) and the second about the ride itself:
http://forums.teamestrogen.com/showthread.php?t=33287
http://forums.teamestrogen.com/showthread.php?t=33697

Catrin, define "light" in this context, please! For me, a good two-day trip would be from ... oh, Catonsville, MD, I guess it is, to York, PA, overnight, then back. It's 40 miles in one direction on the Northern Central Railroad/York Heritage trail. Might not be long enough for you, though!