Paul from Quincy, IL on 8/8/2013 4:16:32 PM:
I don't know if this will help you, but 10 years ago I rode from Cumberland to DC on a road bike with 700 x 25's. It rained 2 of the 3 days, so it wasn't much fun; but the tires did OK.
Now, I have Schwalbe Marathon 20 x 1.5's on my folding bike. I haven't tried the C&O with them. I suspect the hinges would take a beating; but the tires would be OK.