Take a look at a topographic or relief map of Pennsylvania and West Virginia, and you'll happily take the GAP Trail which has a good surface. The C&O Canal towpath is often a muddy mess, and frequently bumpy. However, some parts can be easily avoided. The Western Maryland Rail-Trail is a paved 21 mile trail which is adjacent to one stretch of the towpath. From Whites Ferry, you can cross the Potomac, ride about 4 miles (on a very busy road with fast traffic, but with a wide shoulder IIRC) and get on the W&OD paved rail-trail to get to Arlington, then into DC. It's about 38 miles of trail riding from Leesburg. However, this would bypass the very scenic Great Falls. The towpath from Great Falls into DC generally has a better surface than further away from DC.