If you can handle a paved rail trail , I've been touting the South County trail begins in Bronx goes up to Brewster ny aprox 50 miles.
You can see my posting for this here.
http://www.bikeforums.net/northeast/...tury-ride.html

If your only looking for a dirt trail, create a profile at this site and you get a ton or trails all over usa
Bike Trails, Walking Trails & Trail Maps | TrailLink