Battersea Park

Riverside and Victorian, and more varied than its size suggests: a boating lake, a bandstand, a Peace Pagoda, and enough tree cover to be bearable in August. The Thames path along the north edge is the flattest mile in south London.

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.