Northville to Piseco is about 40 miles if you start from the arch and go to rt 8. It has an “out” at Whitehouse and could be started slightly farther up the trail if you wanted to avoid road walking. If done from Piseco, you’d have a few “outs” if you were slower than you thought,including a major road crossing about 12 mi “trail north” of the arch.
We camped at Lake Durant and hiked to Lake Placid. About 60 miles, relatively flat, but had lots of downed trees in July 2018 that had to be climbed over, under, or around.
We spent 5 days of medium exertion hiking.