The unfinished on-road bits in NJ are pretty good, at least the ones I rode. Trenton gets pretty nasty, I'd cycle through there as quickly as possible. (OTOH, the few people I spoke to there were very nice and friendly.)

There's a recent thread on Delaware River crossings into NJ that mauy be helpful. If you take the offroad Greenway, I'd get off towpath at Schoolhouse Lane, shortly before you get to Bound Brook. (The path gets very rough after that.

You might also google the High Point to Cape May trail. It splits off the Greenway in Bound Brook, and heads north into NY state. It's all on-road from that point, and the roads are mostly very good.