Jill, doing this in reverse would be well nigh impossible - there would be no indication of where to leave the beach.
MapsMe is excellent as it shows all the footpaths . . . . but not the current condition of them.
There is a fabulous boardwalk for a km or two that leads into the Orbitur camping site, which is south of Viana, across the river.
I was fine (going south) until the next boardwalk took me across the river, and then dumped me unceremoniously in the sand dunes.
However, there were distinct paths, obviously used by locals, so I continued on them across the dunes. I then came out onto a vehicle track, where I could either go straight ahead, or turn left and cross back over the river.
I decided to get back across the river, which was a mistake, as I then ended up on the N13. If I’d gone straight (but I had no idea if it was deep soft sand or not) I would have eventually arrived in Amorosa, instead of getting there the long way round, from off the N13, down Av. Atlantico.
I got stuck again after Amorosa, still going south.
I was on the Senda Litoral again, but it was turning inland, back to the N13, and I tried to stay near the coast.
Luis Freixo’s map shows there is a path between Amorosa and Pedra, and so did MapsMe, but I couldn’t find a way onto the beach, which is where the path appeared to be on the maps.
Again I ended up back on the N13. I was now at Castelo de Neiva, so I stayed on the road until I picked up the Caminho da Costa, and then followed that, which was quite easy as there were lots of pilgrims coming towards me.
Jill