A friend & I recently rode from the Pittsburgh area to Washington, DC, on the Great Allegheny Passage and the C&O Canal towpath. We both rode Bike Friday New World Tourists. We each used Primo Comet 20 x 1.5 tires with Kevlar, and they worked well on these two unpaved trails. We rode the trails in early July and completed it in 6 days (we rode a total of 346 miles--about 554 km). We stayed in motels & B&Bs. Everything worked out quite well. We had near-perfect weather the first 4 days, dry & unseasonably cool with low humidity, The last 2 days were extremely humid and much hotter, with a big thunderstorm about 20 miles from Harpers Ferry, our destination our 5th day. The C&O towpath was fairly muddy following the storm but much better the next day.
We both loved the Great Allegheny Passage, and both got a little bored with the scenery along the C&O Canal, which changes little from Cumberland to DC. The towpath surface is not nearly as smooth riding as the GAP Trail, though we knew that beforehand. We happily took advantage of the well-paved Western Maryland Rail-Trail which runs parallel to the towpath in the vicinity of Hancock, Maryland, for about 22 miles at present.
We had both previously visited Frank Lloyd Wright’s Fallingwater near Ohiopyle, but neither of us had visited the lesser-known Wright house, Kentuck Knob, about 2 miles from Ohiopyle in the other direction. The first of those 2 miles was one of the steepest hills I’ve ever seen. The 2nd mile was merely moderately hilly. The house, however, was well worth visiting, though expensive at $16 per person.
We had both hiked through the Pawpaw tunnel in prior years, and I was surprised that we were able to pedal through the tunnel fairly easily with just one not-very-powerful headlight on the lead bike. It's a little bumpy in the western part of the tunnel, however, and our light wasn't strong enough for us to see the bumps.
One thing that surprised us was that most of the other cyclists we saw who were touring like us were even older than we are--and we're not spring chickens. There were practically no young riders. (I'm defining young as under 30 in this context)