Here is a Google shot of Prospect Park.
The roads in the park are generally curly, but I found two bits of roadway that look like they would be suitable for 300 meter or 1000 foot calibration courses. Without local knowledge this is as far as I can go.
I suggest you get a helper and lay down a cal course. You’ll save time overall, even if you use it only once. Given that you are thinking of laying down a marathon, I'll bet you'll need at least two or three days to get it done. That's a lot of trips to Central Park.
The possible areas are shown in yellow below.
