I did the C&O/GAP with ACA a few years back. I Continental Travel Contacts 700x37 on my Trek 520 touring bike, which had room for a set of SGS fenders, and they worked very well. Maybe you have room.

It was a bit muddy for about two day of our trip. Some of the ACA guys said there was no way they would run fenders on the C&O, but the folks who had a skunk stripe up their back were envious.

I debated much before going on the trip, and even considered taking my mountain bike. I felt like I made an excellent choice.