NYC Subway Map
Here is a link to the MTA Subway Map. If you are running out of steam on the NYC side you can see that the A and 1 lines run down most of the west side only a block or two away from the Hudson River Greenway. Access to the Subway begins right after you cross over the GWB in the GWB Bus Terminal building. At that point you have walked about 16 miles from Exchange Pl. - a goodly amount for anyone to be proud of.
Subway Map: http://www.mta.info/nyct/maps/submap.htm