The Putnam and Westchester and rails-to-trail system is actually comprised of 3 trails on the former right-of-way of the New York and Putnam Railroad as follows (north to south):
Putnam Trailway – the northern most 12 mile stretch through the towns of Mahopac and Carmel with plans to extend it north to Brewster. The highlight is the views of the reservoir north of Carmel.
North County Trailway – runs for 21.2 miles from Northern Westchester County south through the towns of Yorktown Heights, Briarcliff Manor, and Mount Pleasant. The path traverses an old railroad bridge that has spectacular views of the New Croton Reservoir at Kitchawan. There are 2 breaks in the trail when you do need to ride on the road. The shoulder is very wide, although the traffic is a 4 lane fast moving road with some on and off ramps to highways that require caution.
South County Trailway – runs for 14.1 miles in Southern Westchester County from Eastview through Elmsford, Ardsley, Hastings on Hudson, south into Yonkers, to the New York City border. There is a break in the path at Elmsford (be sure to look at one of the maps for the best way to avoid a very busy road with no shoulder – we only discovered it on our return). The V. Everit Macy Park is a nice park along a large pond and short but robust waterfall just south of Elmsford. I believe there is a break in the path in Yonkers, although I’ve heard they are completing it – and we have yet to make it this far to check it out for ourselves.
The paved path ends at the entrance of Van Cortlandt Park in the Bronx but there is a dirt bike path through the park (apparently it can get muddy after rainy weather). You can pick up the subway (the 1 or 4 lines) here (bikes are permitted on subways) or go over the Broadway bridge and take some streets over to the Manhattan Waterfront Greenway trail on the West Side. It would be about a 15 mile ride from Van Cortlandt Park down to the lower west side of Manhattan (I map quested it to my office on Hudson Street).
The best end-to-end map of the Putnam and Westchester trails that I’ve found is this one, which also points out the Metro North stations along the way for those who want to take their bikes on the train. The site indicates that the Pleasantville, NY train station is the closest station – 1 mile - to the path: http://www.nycbikemaps.com/maps/north-and-south-county-trailways-bike-map/
This site also has brochures with maps for the Westchester trailways: http://parks.westchestergov.com/index.php?option=com_content&task=view&id=1846&Itemid=4528
The Manhattan Waterfront Greenway is a 32-mile long walking and bicycling path around the island of Manhattan. The majority of the Manhattan Waterfront Greenway is physically separated from auto traffic, although there are several sections that require using city streets with motor traffic. This is a great site for New York City bike routes and paths: http://www.nycbikemaps.com/
So, you could actually ride from Brewster, NY to Manhattan – approximately a 60 mile ride - mostly on bike paths. I have now just added a new item to my bucket list!