As others have said, your credencial will be fine. In terms of the walk out of Porto, you haven't said which route you intend to walk. I'm assuming either the Central or the Coastal, as they tend to follow the northern route out of Porto that a lot of people have complained about. I can't speak to how bad that route out of Porto is, since I didn't walk it. If you choose to walk it you will find that it is part of your Camino experience. Some of those tough experiences we would never have chosen become memories that we cherish. Nevertheless, I chose the other route out of Porto, on the Senda Litoral. As Walli Walker said, you go down to the river from the Cathedral and then west along the river to the ocean and north along the seashore and boardwalk. It joins the Coastal route at Vila do Conde, if that is the route you are walking. If you are planning to walk the Central, from Vila do Conde you can turn right just after you cross the bridge and the entrance to the town and head east along a marked route to the Central, which it joins at Arcos, just before Rates.