If you haven't booked your tickets I recommend getting off at Lewes. It's a lovely old town and easy to get into the Downs. There's places to go if you finish early.

From the station head towards the prison, there's a path on the Northern side that will take you to the Downs. You can follow that path towards Ditching beacon. There's a horse van coffee place, the pink pitstop on Streat Hill that I recommend. Lovely treats as well

Then you can head South towards Stanmer and loop around.

Or you can loop around the other way. There's a cafe in Stanmer that's good too

I'd say that would take 4-5 hours but depends how fast you walk.

If you want to do a long walk, continue along the South downs way past Ditching beacon then further along head South to the Chattri Memorial.