When you get to Caminha, you are on the border with Spain. The city across the bay is A Guarda. There is no easy way to walk there from Caminha. You have to take a ferry from Caminha across the river and then walk a bit into A Guarda. From A Guarda its about 9 miles to Oia. You also lose an hour by going from Portugal into Spain. I actually stayed in Caminha, then since it was Monday, I took the water taxi (5 Euros) across the river and then walked the 2 or 3 miles into A Guarda and stayed the night there. Next morning continued on to Oia. That ferry ride and the hour loss kind of screws up the day. A Guarda doesn't have a lot going for it but it's a good size city with all the services you want and some nice restaurants along the estuary by the ocean.
If it would help, I've put some videos up on youtube that show the day to day walking from Lisbon to Santiago.
I walked from Lisbon to Santiago using the Camino Portuguese taking the coastal route to Santiago. Much of the walk is on highways and backroads (as opposed ...
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They aren't blogs but do show videos of most segments of the path. It's speeded up by a factor of 10. It should at least show you what type of terrain you're in for (if you take the coastal route).