Haven't you asked this question in another sub-forum already. It will only confuse those of us who want to help you to have two active threads on essentially the same thing.
Gronze is a good resource for distances, albeit it is in Spanish but it translates pretty well in Google Chrome most of the time. I walked the Central route, but my understanding is that after Vila do Conde, the two routes are very similar, with very few differences in track alignment. So if you want to do the Littoral, you need to start in Porto, when you will follow it for a day or so until it re-joins the Coastal Route. From there to Vigo, it appears that there are variants that do stay closer to the coast, but whether these are distinct elements of the Littoral route or complementary route section on the Coastal route isn't clear, and probably doesn't matter that much anyway.