Somewhere on the web I stumbled across a map that gave the distance and elevation for various points. Was it NYCC? Anyway, according to that source, it was 6.1 miles around the entire park, though I'd imagine, the distance could vary according to which lane you're in

When I was there on Friday more cyclists seemed to be using the 103rd Street Transverse than the full route. Probably to avoid the Great Hill.

Stacy