I've ridden from Salamanca to Porto, crossing the border around Saucelle and roughly following the Douro valley to the ocean. Don't remember my exact route, you go up and down from the hills into the valley and back as there is no continuous road along the river. There's a lot of climbing involved, all that you can see on the map of course. In the east, it's a fantastic trip, scenery, villages... but it gets less interesting the further west you go. I remember being disappointed about the greater Porto region (not the city itself): densely populated, at times hectic traffic even on the smaller roads to the point that it got stressful (witnessed a few car crashes). So if it were me I'd turn south about halfway the Douro valley (say Vila Nova de Foz) and head in SW direction through the Sierra de Estrela national park and 'old' countryside, which is really lovely in Portugal. Best part along the ocean IMO is under Lissabon anyway, SW Alentejo NP (look up rota vicentina).
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