Hey, I live in UWS too and going over the GWB to 9W is the best place for longer rides. I think biking around the park is way too busy and slow, especially on weekends. Let me know if you have any other questions or need help finding it.
I find that Central Park's only okay for biking if you're in there very early. The GWB and 9W is the typical ride, but if you want something a little different it might be worth checking out the New York Cycle Club - they do a lot of rides of all levels all over the place (and they're free).