If you have the time, start at the Cathedral and walk down to the river. Follow the river to the ocean, and then walk north up to Matosinhos. I think it is about 12-13 kms. You can then take the metro from Matosinhos back into central Porto for the rest of the day/evening, and back out again the next morning. I think it would be a good first day - scenic, not too long, and a good warm up
In 2013, I started at the Cathedral and walked through the city. I ended at Vilar de Pinheiro, which is south of Vila do Conde. If you are a stronger walker, and don't mind longer distances, I guess Vila do Conde would be possible. I just Googled it, and it says that Porto airport to Vila do Conde is about 20km. If the Cathedral to the airport is another 15-20 kms, I am guessing it would be 35-40 km overall. That is outside my comfort zone, especially on the first day.