The maps that Dillbag is talking about are the NY/NJ Trail Conference Maps. You can get them a bunch of places here in town, and probably even a couple of places by you. You will definitely need these maps to calculate a distance. If you try to do it off of some half-assed satellite map, you find yourself in the middle of nowhere on your 21st mile with no water, etc.
Another good thing to do would be to call the running store in New Paltz, Peak Performance. The people who run the store also coach the SUNY New Paltz Cross Country team, so they know all the trails and are pretty accomplished runners themselves. Of course, you could stop in there to buy the maps, and talk to them about a good route.
I'll also make one other suggestion if you're game: Someone I know from my high school days moved up here recently and has opened up a B&B which is dedicated to training long distance runners and tri-atheletes (I'm actually editing a piece about his place right now). If you're interested in some very top end coaching for the race, PM me, and I'll put you in touch. A friend of mine who's been running the NYC Marathon for the last 20+ years stayed with him this weekend, and he was pretty impressed. Afterwards, he told me, "For 20 years, I've been doing just about everything wrong".