In addition to Blue Mountain and Bethpage (which is pretty boring, actually- Stillwell Woods and the Greenbelt are a bit more exciting if you're riding out that way), there's Sprain Ridge Park just north of Yonkers, Graham Hills Park in Pleasantville and Ringwood State Park in Passiac County, New Jersey. All of these are as far as or closer than Blue Mountain to NYC. If you want to venture out a bit further, there's Ninham Forest and Fahnestock State Park in Putnam county, NY and Wawayanda State Park and Mountain Creek/Diablo Freeride Park out in Sussex County, NJ (they've got lift-accessed XC trails). There are a bunch of other parks in NJ with MTB trails, but trust me- none match Ringwood or Wawayanda.
Check out www.wmba.org for maps of parks in Westchester county, NY.
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