I am going to be the contrarian here. I think it is too many miles a day for someone of your limited experience. 14 miles a day id pretty good for 5 days a week but you are talking about bumping that up to 70+ in a day. That is an enormous difference.

Trail ridding is not like road ridding. I would much rather do 60 miles on the roads with hills than 60 miles on a trail like the GAP or C&O. Don't let the fact that it is flat fool you. And if you get some rain, it can be down right miserable ridding on those trails.

Why not do something a bit ambitious like just riding from Pittsburgh to Cumberland for your first time? You can extend that trip by taking the Montour Trail or exploring Pittsburgh a bit. I can show you how you can even do both if you want. You could also take the train back to Pittsburgh.

Or you can just ride to Martinsbug or Harpers Ferry then take the train into DC. That would get you at least part of the C&O.

There are lots of options to not pack so much into your vacation and still have a great time.

Let me know if you need any help at the Pittsburgh end or want information on the Montour Trail. In my opinion the with the exception of the Ohiopyle area, the Montour is every bit as beautiful as the GAP. And certainly a lot prettier than the C&O.