Travel from Porto to Santander Spain

'the train travel. It seems very indirect.' - Yes.

You can perhaps knock a couple of hours off the journey time and less changes by bus.
Oporto to Santiago with autna.com and onward with alsa.es Check for yourself.
There are probably a couple of other bus permutations which probably involve three or four changes and more time.

Or fly maybe, but I know nothing of it.

Bus seems the way to go as the train requires many changes and with luck takes 14 1/2 hours. The bus takes just over 12 with a change in Santiago de Compestela. Having visited SdeC, you might want to consider an overnight there as it's a great town and the end of the Camino with lots of interesting sights. For more info on the buses and trains, click Porto>Santander

The train system in Spain is not as developed as in out parts of Europe and does not connect well with the Portugal train system. But Spain has a terrific bus system - cheap and convenient - because of that. When we are in Spain we almost always think bus first, train second - some exceptions, of course. Buses in Spain are very nice so don't hesitate to use them.

This great and helpful information. Thank you all. The Rome2rio site is neat. What would renting a car like in Spain?

Renting a car can be a good value for four travelers, but it will cost more if you pick up the car in one country and drop it in the other. There are a couple of direct trains per day from Porto to Vigo, then you could pick up the car at Vigo rail station and drop it in Santander. Try a price quote through Auto Europe.