I walked out of Porto along the river (keeping it on my left) towards the ocean. You will not see any yellow arrows until Matosinhos. The first couple of days of the Littoral route is quite urban but I quite enjoyed it. From there to Caminha, I walked a blend of the Littoral and Coastal routes. The arrows out of Seixas (after Caminha) lead back up into the forests, apparently that is the original route. My advice is to ignore the arrows and just stay on the river path, a lovely, easy walk. The town of Vila Nova de Cerveira is slightly off route (as in, you need to leave the river path and walk an short distance into the town) but totally worth a stop for coffee & a pastry!
The more reading on forums, and when you receive your guides books, the more it will all make sense. Happy researching and have a wonderful journey.