Length: 15 km (9.3 miles). Toughness: 2/10
Meet up for an 11:15 start in the ticket hall of Wimbledon station (in TfL Zone 3). It's the terminus of one of the branches of the District line and also has frequent suburban services from Waterloo; if coming this way, take a train no later than the 10:57 to Hampton Court, arriving at 11:13.
You can return from either Putney mainline station (Zones 2/3; five trains an hour to Waterloo) or Putney Bridge (Zone 2), on the Wimbledon branch of the District line.
For a shorter walk you could drop out after 11½ km at Barnes station (Zone 3; same service as Putney). See the walk document if you want suggestions for a longer walk.
This new walk was inspired by the waymarked Beverley Brook Walk, although with a different start to reduce its proximity to the noisy A3. It goes through Cannizaro Park, Wimbledon Common, Richmond Park, Barnes Common and Lower Putney Common, with stretches alongside both Beverley Brook and the Thames. Note that less than half of this urban walk is on hard surfaces and one stretch (alongside Beverley Brook in Wimbledon Common) can be relied upon for some nice gloopy spots in winter, so come prepared.
The walk would delight members of the Temperance Society as there are hardly any pubs en route. If you want to break for lunch there are picnic tables and a café near the Roehampton Gate exit from Richmond Park, just past the halfway mark. At journey's end there are several refreshment places near Putney Bridge, with a Wetherspoon's on the riverfront being the favoured watering-hole on other SWC walks ending there.
You'll need to bring the directions from the L=swc.391 page.