I believe that a Croydon Stations - Tonbridge
Route Not Via London Off-Peak Day Return at £9.80 is valid for your journey.
Edenbridge Town - Edenbridge is a recognised walking interchange and the route via the Edenbridge stations (24.5 miles) is shorter than the shortest route entirely by rail (27.25 miles), which is via Sevenoaks, Orpington, Bromley South, Beckenham Junction, Crystal Palace and Norwood Junction. (Source:
here)
So finish short on the outward at Oxted and start short on the return at Sevenoaks.
Edit: I see that soil was thinking along similar lines.