Lisbon to Porto Bike Routes? Which on is preferred?

Wayne of LaVerne said:

Lisbon to Porto: Advantages of each bike route option? The coastal route does not appear to be the most used. Why?

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I don't know anything about cycling but there is no real 'coastal route' from Lisbon to Porto that's actually a Camino de Santiago. There's the Caminho do Mar which hits the coast briefly at/around Cascais and then heads inland to Fátima, or else you can just DIY it along the coast. There will certainly be roads that you can presumably cycle all the way along the coast, but it isn't a camino.

Edit: When people refer to the Portuguese coastal route, they mean starting from Porto.