Nothing to stop you walking out to the coast from the airport (a good 90 minute walk out to Lavra/Labruge) but not a very inspiring route.
You could also just take the metro into town (25 mins to Trinidad), walk straight down to the cathedral to get a stamp, load up with breakfast and start straight out of Porto.. out to the coast at Foz and head north..
Or take the metro and go a few stops to Senhora da Hora and change there for Matasinhos (Mercado stop) to start from there, it will give you 10km head start out of Porto.
Here's a photo from just a couple of hours ago showing the coastline at Vila do Conde, a few km to the north. This is where most folk leave the coast if taking the central way..