51 and only a few adventures left??? You are younger than me and I have plenty left in me. You are talking about the Great Allegheny Passage. While some areas are a bit remote you are never a day from a town. There is plenty of places to get help along the way with plenty of riders to lend a hand. And there is plenty of beauty along the way too.

A 50-60 mile day is doable. Some folks will do the whole thing all at once in 36 hours (and that is all the way to DC), The trail is mostly flat with less than 3% grade at any point. Very easy but I find it a bit tiring as it is too flat. I do much better on rolling hills. IMHO it is easier to start in Pittsburgh. The trail does continue to DC. Starting in Cumberland it is know as the C&O trail. Not as good as a surface as the GAP. Lots of roots and ruts. If it rains there will be plenty of mud.

The trail does start from downtown Pittsburgh and is complete all the way to Mckeesport. This section is paved the whole way.

An option would be to take the Montour Trail from the airport (assuming you are flying in or dropping off a rental). The Montour has sections that rival much of the GAP IMHO. You can also take that trail to Mckeesport but is nearly twice the distance and you would miss the city. The bike museum in the city is well worth seeing. A compromise would be to take the Monour to Library, PA then take the transit system (The T) to the city. This is the prettiest part of the Montour anyway.

PM me if you are serious about the trip. I can act as tour guide, SAG or a place to stay at the Pittsburgh end. I live very close to the airport.

You don't have to camp but there are plenty of good spots to camp if you want.

Just some places to stay:

Yough Shore Inn, in Little Boston just outside of Mckeesport. Lots of fun, I highly recommend.
The motel off the main street in Ohiopyle (I can get the name for you later if needed). FYI Ohiopyle will be expensive.
Connellsville comes before Ohiopyle and I have yet to find a place near the trail so you are probably better pushing on to Ohiopyle and should plan accordingly.
The hostel in Rockwood, PA. Very clean and cheap.

To get here, I would rent a car and drop it at the airport. You could leave your car in Cumberland or DC or maybe just take the train from DC back home.