This walk in the Darenth Valley has several options.
I've homed in on one, the Short Walk, omitting Otford (option c) but suit yourself. This stops for lunch in Shoreham.
Trains: get the 09:45 from Victoria arriving Eynsford at 10:35 (this train visits quite a few stations on the way, including Denmark Hill 09:54, Peckham Rye 09:57 and Catford 10:06)..
Trains return from Eynsford at xx:41
Trains return from Shoreham at xx:38 (for an even shorter walk).
Get a return to Eynsford or, to finish there, Shoreham (the next stop along).
Lunch:
In Shoreham: The Samuel Palmer (01959-525442), an up-market pub / restaurant on the route. There's a coffee shop at The Mount Vineyard (01959-524008;). Also, in the village, the Kings Arms (01959-523100) on Church Street, and The Crown (01959-522903) at the northern end of the High Street..
Tea
The Plough Inn (01322-862281) has a fine riverside location. On the main road towards the station there is The Malt Shovel (01322-862164) The detour towards the Castle passes The Five Bells (01322-863135) and the Castle Hotel (01322-633917).
Directions: here
click on option c T=swc.59.c
A detour near the end will take you to the ruins of Eynsford Castle Open till 4.