Don't hesitate to use Prospect Park. There is plenty of room for everyone there. Some sections may have people crossing the road, but it is never like Central Park. Just be careful on the downhill as there is a crossing point at the bottom. Pedestrians are usually aware though.

It is 3 lanes wide, with one lane devoted to runners and cyclists for when the park is open to traffic, otherwise bike are free to use the other lanes. Mostly walkers stay off the road.