If you wanna to leave the city, Prospect park in Brooklyn offers a nice 4 mile loop. I train there and CP and find more peletons to hop into in Prospect Park, if that's your sort of thing. There is a small hill and also a fair downhill, but more importantly it is less crowded than central park. Also there is a team that trains there a lot, I forget their website, but they race there as well. You should be able to find it on active.com or by searching google. Also, Harrimon state park north of the city is GREAT! I do bi(du)athlons there in the warmer months. You migth be able to carpool with someone too... some killer hills in Harrimon, and a long stretch to get up to 50-60 mph easily.
Check NYRRC.org to see when the races (running) are, you might want to avoid central park then, thousands of runners (myself included) glog up the park and often forget (myself excluded) to watch the bike lane.