The C&O can be rugged in places, especially the western portion. It has not been upgraded above what it was for the mules that pulled the canal boats. Near Hancock however, the paved Western Maryland Rail trail parallels the C&O and was a welcome relief.
We opted to divert off the trail for a small portion to take in the Battlefield at Antietam and some of the surrounding communities.
Near DC the path is well maintained crushed limestone. It becomes busier with cyclists as well as walkers and runners. The next thing you know you pop out of the tree and your are in Georgetown.
I would do this trail again and very much recommend it. As others have said the grades on the roads in the Allegheny's are extremely challenging if that is what you want.