This is the old croton aqueduct trail, which follows the old croton aqueduct from Croton Reservoir, NY south to finish at 42nd and 5th Ave, behind the NY Library (At Bryant Park). The trail follows the route of the actual aqueduct, which originally was the only supply of freshwater to the manhattan area. It is maintained by the Friends of Old Croton Aqueduct. The route can most accurately be found in the 2 maps that the Friends of the Aqueduct sell https://aqueduct.org/maps-and-access
The Friends of Old Croton Aqueduct have also implemented a cool patch and certificate system, whereby they give you a 26mi and 41mi patch for completing the first section of the trail (Croton to just south of Yonkers, NY) and the entire trail (Croton, NY to 42nd street Bryant park). Details are on their website (listed above)
The route was probably first run in its entirety by Peter Bakwin on May 24, 2007. His report is here.