hmm, that embedded google map renders with car directions in my browser, showing a route right up the Henry Hudson Pkwy, off ramp of the GW, etc., which obviously wouldn't work on a bike. Funnily enough, if you switch google maps to 'bicycle' it takes a route pretty much directly across Manhattan and into the Bronx, then up University Ave. The bottom line is, you really can't avoid lots of traffic until you get into Van Cortlandt Park.

The Putnam trail runs right up the middle of the park. Here is someone's ride starting in the northern part of the park and leading on to the South County Trail It must of been dry for a while when this was shot. I have had some unpleasant rides on the Old Putnam Trail leading up the SCT.

My recommendation remains B'way to Yonkers, then the Old Croton Trailway to Hastings.