You can take Metro North to Bear Mountain.
You can also take NJ Transit to the Peapack NJ station. Peapack is centrally located to a lot of nice climbs including
Black River, Pennbrook Rd (aka Jacob's
Ladder) and a lot more. You can easily do 3,500-5,000 ft of elevation over a 40-50 mile ride.
If you're REALLY serious about breaking yourself then ride from Peapack out to the Washington Twp area that's between the Delaware River and Route 57. There are climbs, like
Fiddler's Elbow, that ramp up to
20%+ inclines. You can find more info by Google searching on "Hillier Than Thou" routes. Most of the NJ monsters are on that ride.