I just finished a three day tour from Baltimore to DC then onto the C&O and up to Harper's Ferry, across South Mountain to Cunningham Falls SP then back to Baltimore.
The C&O was incredibly muddy,every twenty feet or so I had to go through or around big muddies. Bike caked with mud etc. I only encountered a few long haulers but none stopped to chat, probably too concentrated on getting miles done. The trail can also be quite monotonous. Didn't see anybody at any of the HB campsites.
The roads around Antietam were very nice and the hills were a nice change form the monotony of the trail. Can't imagine doing the Cumberland to DC length, at least not without an i-pod.