Length: 15½ km (9.6 miles), or 13¼ km (8.2 miles) if finishing at Morden. Toughness: 1/10
10:38 Dorking train from Victoria (Clapham Jct 10:45, Balham 10:50), arriving Mitcham Junction (TfL Zone 4) at 11:00. There are no Thameslink trains to Mitcham Jct this Sunday, but you could also get there from Wimbledon or Croydon via Tramlink. Use Oyster or contactless (or buy a Travelcard) as you'll be returning on the Northern line.
NB. If you're on a later train and don't fancy a brisk walk to catch up, you could go one stop further and join the walk from Hackbridge station: see para [4] in the directions.
Much of this suburban walk is along firm paths but don't expect it to be completely mud-free. In the first section there's a long stretch alongside what's supposed to be “London's next major nature reserve” but the promised restoration of the Beddington Farmlands site has been painfully slow. After that the rest of the morning leg is a loop through Beddington Park to Carshalton, with several places to choose from if you want a pub lunch.
The afternoon leg follows the Wandle Trail, a riverside path taking in some attractive parkland and more local nature reserves. The National Trust's Potting Shed café in Morden Hall Park makes a convenient tea stop. It's another half hour to Colliers Wood, but you could choose to call it a day and finish at Morden.
You'll need to bring the directions from the L=swc.273 page.