I walked around for over 3 hours, trying to explore the most of the park, which I did.
Just to explain how big the park is, it goes from 110th street to 59th street.
When I tried to take the subway back, someone could get in the subway, and I used my metrocard to help them, and since I used it once, I couldn´t use it myself to get in. So I had to buy a one ride card to get in.