Been a bunch of threads here.
http://bike-pgh.org/bbpress/topic/things-to-know-about-the-c-038-o-towpath
http://bike-pgh.org/bbpress/topic/dc-bike-trip
Here's a quote from what I wrote:
There are ways to bipass parts of the GAP trail to Cumberland, I guess, but the trail always seemed fine to me.
Not so the C&O.
It was originally conceived as a hiking trail and there are people strongly committed to historic accuracy and non-development, i.e mud not crushed limestone.
On the C&O, when it rains, it becomes apparent that you are riding on century-old mule shit. When the rain stops, you will learn why they use dead horses to make glue.
There are parts of the C&O where it would be hard to avoid the trail - particularly the first few miles out of Cumberland, with 1000+ ft climbs. Other parts, it is pretty good to take the roads.
Around Hancock, there is 20 miles of paved rail-trail that is parallel to the C&O and often in sight of it.
From Williamsport through the Antietam Battlefield, Sharpsburg, and to Antietam Creek is a lovely ride on the roads - and if you "stay on the trail" you have to take a hilly detour anyhow.
There is a ferry ($1 American) over the Potomac near Leesburg, VA, then a paved rail trail from Leesburg to DC.
All in all I've done about 600 miles "on" the C&O, but that was really about 370 on the trail and 230 on roads. I don't regret that one bit. There is about 20 miles of the C&O that I missed -both trips, both directions.
Maybe heaven is on that 20 miles, I dunno. Doubt it.
There seem to be plenty of places to stop for food, water, beer, or beds on the GAP, but far fewer on the C&O. Plenty of places to camp on the C&O, but if you are doing B&B's and motels, you have to plan.
Aside from the trail - which is a hiking trail after all - the canal is next to it. 300 miles of mesquito-breeding heaven. You want to avoid insect season - which starts in june.
The trail isn't bad if you don't get hard rains.
There is a tremendous amount of cool wildlife - turtles sunning on logs, blue heron, hoot owls, deer, snakes, red-winged blackbirds, "black-winged redbirds" too, a bad-tempered goose with 19 (nine-FN-teen) goslings following her.
Boredom wasn't an issue for me at all. Muck-filled potholes at the bottom of mudpuddles? Yeah. Boredom? No.
Take good maps.