I ride the greenway every day. Not for commuting. I ride up to the George Washington Bridge & down to the trade center. Its commuting during peak hours, but off hours you can ride for fitness without worrying too much about others. Central park is a 6 mile loop with 3 hills, 1 of which is pretty steep for Manhattan standards. Avoid biking over the Brooklyn Bridge if possible, stick to the Manahttan & Williamsburg bridge if going to Brooklyn. I would bring a beater bike if you have one and want to lock it up outside. Make sure to lock through the frame and rear wheel and also lock the front wheel. People will steal bikes in crowded streets in day light (look on youtube).
The greenway + Central Park loop is about 26 miles, so Its easy to get miles in without dealing with cars & traffic lights. I average 17-20mph.
here's a snapshot of a typical greenway ride for me:
Typical Central Park Loop for me:
