Are you talking about the greenway the goes around the rim of Manhattan? It's supposed to be a master plan for a collection of connecting greenways that will criss-cross and ring the island, but I think the waterfront is the only part that is mostly complete.
Been on the West Side Greenway a couple of times (blowing off extra energy after the Bike New York tour.) From the Battery to around 133 st where it detours around a grocery store parking lot. The lower part isn't very green, and its very crowded on weekends, but it is a nice ride. Very, very flat. Gets better when it gets to Riverside Park, around 59th st.