I had a plan last year in the spring to do both the GAP and C&O in about 5 1/2 days. Plan was to pass through Cumberland partway through day 3. But even before I got to Pittsburgh, I was finding the mileage goals I had set for myself to be a challenge and to be taking a lot of the fun out of the ride. Instead I changed my plans, and gave myself 4 days to just do the GAP. It was very enjoyable. My longest day was 50 miles, and it was getting dark when I got to camp. I'm not a fast rider to start with, but knowing I had low mileage goals meant I could stop, read the information signs, take in the view, have leisurely meals, etc. Unlike some of the other riders, I enjoyed the urban parts of the trail as well as the wilder parts. I would try to squeeze at least one more day into your schedule and do the whole trail. Make the first day your longest ride of up to 60 miles, spend the night in Roundbottom or Connellsville or that KOA that others seem to have a higher opinion of than me (but then I sleep in a hammock, and that campsite appeared to be a field). Then on to Rockwood. Then you've got plenty of time to get into Cumberland for the train (7ish, I think?) on the last day. Plus the 2nd half in an easy, downhill ride. Of course if your first day involves driving to Pittsburgh, you may want to make the 2nd day the longer one.