I don't think the D&R is in PA. While there is a canal path on the PA side, I don't think it's part of the D&R system:
http://www.dandrcanal.com/pdf/parkmap.pdf
I have a route I use routinely to get to/from New Hope, where you can cross the river to Lambertville to pick up the D&R. It actually starts out of town in Conshohocken. The start is easily reached from center city via streets and the Schuykill River Trail. It's on Bikely, but the sight seems to be down right now. Send me a PM and I can give you the link later.
Can't help you with a route from the D&R to Princeton.