Afflatus -- I rode the entire canal over 4 days in July on a charity ride. I used a hybrid with 700x35 tires and found it very comfortable. Many others rode mountain bikes which might have been nicer in the muddy bits, but otherwise were a little slower. There is one on the road detour (since there is no towpath in that section) if that tips the scales against mountain bikes. Either a hybrid or a mountian bike would work quite reasonably, I would not suggest a road bike since trail conditions can vary a good bit.

From Cumberland to G'town the canal has an unnoticable downhill slope. The only place you will see a hill is at the locks and those are only about 8-10 feet high. The on road detour has a few hills but nothing major. The only hills you are likely to encounter will be if you leave the canal for hotels or campsites where you have to climb out of the valley. Other websites will give you many more specifics on trail conditions, but generally I found the western end of the towpath to be fairly smooth and pleasant riding. The section from about mile 50 to mile 20 can be the muddiest, but obviously the whole towpath is affected by recent weather. From about mile 20 to G'town the trail sees a lot of wear and I found the surface to be some of the bumpiest and roughest. Some of that may have been caused by the flooding and huge rains at the end of June so that might not be an issue once reapairs are made.

As you probably already know practice rides on dirt and gravel will make the towpath seem easier if you are used to the extra bumps and vibration. Enjoy your ride, it is certainly a pleasant tour to to for a good cause.