Hackney Marshes

Flat, enormous, and almost entirely given over to football: for most of the last century this held more pitches than anywhere else in Europe, and on a Sunday morning you can still hear all of them at once. Parts of it are levelled on rubble cleared after the Blitz. The Lea runs along the eastern edge, which is the quieter walk of the two.

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.