I agree with everything above, especially dwmckee's notes. 28mm's are sufficient for the GAP but I was very uncomfortable on the C&O and wish I would have had 32mm or wider (mostly for the lower tire pressure, my hands took a beating). I relied on the wells going through the first time, but being finicky, I made sure to fill up three bottles of water the next time through and just get through the stretch between Hancock and Cumberland in one shot. Some of the camp sites are nice (generally closer to Hancock where there was easy river access to cool off) while others feel like you are lost in mosquito infested wilderness.

The bike shop in Hancock is a nice stopping point in that it has a $10/day shelter with a fire pit, showers, etc. I'd make it a point to stop there for a night as opposed to camping in the woods. Make sure you have a light to get through the Paw Paw Tunnel (there's a town nearby, across the river, Paw Paw, WV., if you need supplies).