London Fields

Common land since at least the sixteenth century, when it was where drovers rested livestock on the last night before the City. Now it is a long green rectangle through the middle of Hackney with a lido at one end. Flat, busy, and more thoroughfare than destination, which suits an evening loop.

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.