One I'm familiar with is the Crab & Winkle trail from Canterbury - Whitstable (c.8 miles). Some of it runs along the old line of the same name that originally ran from Canterbury West & was the 2nd railway to open in the UK.
The Downs link is 36 miles long from Guildford to Shoreham & includes large sections of both the Cranleigh & Steyning line, with some intact stations.
As mentioned earlier,
DEFINITELY do the Three Bridge - East Grinstead (7 miles), East Grinstead - Tunbridge Wells (14 miles) route. I live in East Grinstead & the route is stunning in spring/summer when all the wildlife/fauna are out.
Once hotels open up, I recommend staying in Tunbridge Wells & then riding the Cuckoo Trail (11 miles) from Heathfield. Horam station is still there with BR Southern region totems & gas lighting fitments etc.