Prospect Park is good. I've been running and biking there for about 10 years. Spring/Summer weekends do tend to get crowded so i usually go early in the morning (6:30-7 ish) and aside from the races, it's fairly quiet. As someone said, those guys put your speed in perspective. I was cruising at around 20mph on the flat parts last weekend and they blew past me like i was standing still! love being out there in the winter too on those cold, sunny days. I don't even ride in/to Central Park anymore...