I think the best trails out there are East Setauket and Glacier Ridge. You won't find a single rock out there though. But miles of smooth single track. Not much extended climbing either, some shorter power climbing. Check www.climbonline.org for directions. Most of the trails are really well marked (not Bethpage).
I'm not sure where you saw the rain is missing Long Island though .. Looks pretty saturated over the weekend:
Glacier Ridge is absolutely awesome, you can ride the whole thing in an afternoon & every foot of trail is fun, love that place.