Central Park is 3.41 square km on the Upper West Side of NYC. It originally opened in 1857 and is popular with walkers, runner, and cyclists.
You can easily spend a few hours walking (or running or cycling) through the park.
The maps in the area aren't very good, we spent a significant amount of time finding Cat Hill - or Still Hill (we were only successful because I knew you ran past it and therefore if you find the runners you will find the cat).
I'd love to see it in the spring when everything is green and flowering, but it was still worth the trip in the winter.