I do not do dog parks, but my guys have a fenced in yard which Cheli LOVES to run crazy in. I am very lucky as he knows his own limits and will give himself a time out. He runs tothe back dorr and looks over at me -- I open the door he goes in gets some water then lays on the tile floor. When he is rested he will come back tothe door and scratch to come out.
We don't do leashless parks either, but Buster does a lot of racing around here. Ruggles will just lay on the floor and Buster runs circles around him, leaps over him, playing hard. Buster stops when he gets tired!