Tooting Bec Common
This large open space in South London has a cycling trail running through the western side of the park. There's also miles of walking trails taking you to avenues of trees, acid grassland and a small pond with wildlife. The common covers 92 hectares (221 acres) so you can easily spend a couple of hours strolling around. There's also a cafe, tennis courts and an athletics track. A park run takes place every Saturday at 9:00am at the time of writing. Tooting Bec cafe is situated in a historic Victorian building and is a nice place to sit outside and take on some refreshments after your exercise.
The Capital Ring long distance walking route runs through the common so you can pick this up to extend your walk to either Wandsworth Common or Streatham Common which are both nearby. If you're on a bike then on road TFL cycle routes will take you to Clapham Common and Wandsworth Common from Tooting Common.
The whole of this route is suitable for walkers while cyclists can ride the shared surfaced path on the common's western side.