I've done the westernmost 150 or so miles of the Erie towpath twice and could easily count the number of walkers I saw on my fingers, except for the short stretch through Fairport which is sort of a multi-use promenade where you must dismount & walk for a few hundred yards anyway.

For me the more rural the better as far as riding, so I avoid 20 as much as possible. If you like hills and are not averse to finding yourself on a gravel road occasionally, I never found myself going wrong on any of the little roads in the hills south of 20 in the Finger Lakes or the Cortland-Chenango-Madison county area to the east. I don't have a GPS and have found the DeLorme atlas to be great.....I just cut out the relevant pages rather that carry the whole book. Remember you did say you liked hills.