Bklynmom,
A Brooklynite cyclist friend of mine says that Prospect Park is great for hill training if you approach it correctly. He says you don't need anything steeper to train on until the hill is easy to climb in a very high gear. He suggests using Center Drive to make the laps shorter therefore decreasing the time between climbs.
What gear are you climbing that hill at? And, how comfortable is it there? How fast can you climb it? Specifically, what is the lowest mph you dip down to on the climb?