Hyde Park

Taken by Henry VIII from Westminster Abbey in 1536 to hunt deer in, and opened to the public a century later by Charles I. Three hundred and fifty acres in the middle of London, which makes it the easiest big walk to reach and the busiest one once you are there. The Serpentine runs through it, and Kensington Gardens carries straight on at the west end, so it is a longer walk than the map suggests.

Hapoo counts the walk you were doing anyway

Read off your phone's motion sensor after the fact, so there is no start button and nothing running in your pocket. Characters turn up at real park gates across London, this one included.