My husband and a friend completed the McKeesport-DC ride on Saturday. They had one day off and rode for 8 days. It has been very muddy and the Potomac was over the trail in a few places. They got REALLY muddy. He did not have a rear fender and wished for it. He said the riding was nicer on the Great Allegheny Passage. It rained so much, though, that instead of camping, they stayed in B&B's along the route. The book recommended above is definitely a good one to have. After getting to DC, they rode the Washington and Old Dominion to Leesburg, VA. He then went to Front Royal and got on the Skyline Drive and is now on the Blue Ridge Parkway and headed for North Carolina!!
Despite the weather, he really loved the ride. He is riding a Specialized Tri-cross bike outfitted with four panniers. The tires are 32's and he was glad for them in the gravel of the C&O which is much more rutted and less maintained than the GAP.
Have fun. It is a great ride! When he gets back, he will post a trip journal on Crazy Guy on a Bike. He is not carrying a computer so will do the whole thing when he returns.