We live 5 minutes away from Prospect Park by bike. It's roughly a three mile loop with a couple of hills, enough to get some speed going down, and long enough to feel the burn on the way up. It has a one way two lane road for cars and another set for cyclists and pedestrians. However, be careful of going too fast. It's a pretty good place to train, and more importantly, unwind.
I'm usually at the park early in the morning, weather permitting, or later in the afternoon on weekdays. I ride anywhere from one to two hours, putting in 20-30 miles.
It gets a little too crazy over the weekend, but the park is closed to traffic. It's good for going at your own pace since there's a lot of room to ride. I often see members of the Kissena Bicycle Club spinning in their pace lines pushing 25 mph on the flats, so there's definitely competition out there to push me along. I probably wouldn't have gotten back on the saddle so hard if it weren't for this park.