if it’s offroad you’re after and can get to the river, try following these (terrible) directions.
On to the Thames path at Putney bridge
Left after Beverly brook
Cross Putney lower common (cheeky)
Across Mill Hill road into the woods
Queens ride to gypsy lane
Gypsy lane to Putney park lane (dirt track)
Into Putney heath (cheeky woodland)
Under the A3
Trundle through Wimbledon common for a while
Turn right at some point (through a car park?)
down a big hill to Beverly brook again
follow the brook back towards Richmond park
Offroad lap of Richmond park
Drop out of the park at Richmond gate
Head down to the river
Follow the river back round to Putney
Should be 35km + with very little road.
or have a look at a roughly similar route (missing the return via the river and starting with the river, Barnes to Putney) on Strava here
It’s not that taxing, I did it fixed/brakeless but it does get the wind in your hair and the miles in your legs.