I do the NCR trail all the time, most recently in August - really enjoy it. Years ago we had to shelter in the tunnel when a small tornado went through the area!
There is a bike shop and plenty of places to get food in New Freedom, more food in Glen Rock. There used to be a restaurant in 7 valleys at the 13 mile mark from the PA line but it is closed now. Lots of food in York - if you do it on Saturday, go to the York Market a block east of the terminus park, great food. Easy to carry enough snacks to get you through the middle.
The York Heritage Trail surface is much better than the C&O, the the northern end of the MD sections are closer to the C&O kind of surface but most of the southern part is still more crushed limestone that packed dirt and tree roots like much of the Towpath - I'd derate the mileage to predict how you'd feel on the C&O.
Checking bike fit as others have suggested is the first thing to do about having back pain. Another thing: if you are riding a bike with flat bars you have limited hand positions (compared to drop bars). On the hybrid bike I ride on unpaved paths, I added bar ends to get another hand position, gives you a change of angles that seems to help.
Any gloves will be better than no gloves when it is cold! I stick with breathable gloves because no matter how cold it is my hands get sweaty - for temperatures above 40 it is more about blocking most of the wind and trapping some heat for me, vs. staying completely hermetically sealed! But, everyone is different.
MAK - if you call the park office (410-592-2897) in advance, you can park overnight in any of the NCR (I guess it is really the Torrey Brown trail in MD, York Heritage Trail in PA) parking lots in MD, like the big lot on Paper Mill Road. I think you are supposed to call 2 weeks in advance, when I did it years ago I called a few days ahead and it was fine.