Doing the Towpath in 2 days is doable if it hasn't rained for a few weeks before, tough if not. Using the WMRR for the 26 miles or so now paved east and west of Hancock will give you a good break and increase average speed.
I've done Cumberland to Ohiopyle on the GAP, that is an easy day. The rest of the 50 or so miles on the GAP is pretty easy - big rains may cause issues around Connelville. I've never done the Montour part, have no meaningful input there.
I've done the trails in/out of Columbus and in the Dayton/Xenia area - many are along creeks and have flooding after hard rains but are fine when just normal rainfall.