Take a bus to the areas east of the airport  (try the buses 25, 45, 81) There, walk to the Galp gas station next to it . This gas station is right before the highway towards North A-1, headed to Porto, Coimbra, Aveiro, etc..
Check out all the gas station on A-1 highway 
Option 1: Go to Alcântara (Alcântara train station or bus e.g. 720, 52, 24, 773) walk to the acess of the highway that leads you directly to the bridge over the river towards south via highway A-2. Walk up the access and imediately before the highway is a little parking space where cars can stop . There is often a lot of traffic on the bridge so the cars aren't going that fast.
Option 2: Take the Fertagus train, at Entrecampos or Sete Rios train statio for example, accross the river and get off at Foros de Amora . From there just walk under the A-2 highway and get to the gas station right next to it. This will be the first gas station on this highway and from there you can ask people which way they headed and take off towards south or east.
- BRISA has information on all the highways in Portugal and their exits and gas stations.