Just did a part of that ride in reverse this past year, starting from DC.
The C&O and GAP are amazing trails, definitely spoil touring cyclists. Upon nearing Pittsburg, we left the GAP in West Newton to hit backroads up and around the city. A near hundred mile day got us around the city and connected with the Armstrong Trail, a growing rail trail north of the Pittsburg.
From there it was a bit of pavement pounding to get around Buffalo, connecting with the Erie Canal trail, and eventually, Fort Kent, Maine.
Oh, and the state bike routes are pretty handy as well, though I never used them myself:
http://pennshare.maps.arcgis.com/app...56bd2d2a85ec46