Hi, Larry,
I haven't been on the parts of the Camino Portugues that the others describe north of Porto. But if you think you're going to stick with it from Lisbon, I have a suggestion about when you leave the airport.
The airport exit road leads to a big traffic circle. There is no N-10 option, the signs point you to the super-highways or into other parts of the city proper. I was there yesterday and I think that the best option is to make your way to the river. One of the options at that circle is a street with signs directing you to Parque das Nacoes (frequently abbreviated as Pq. Nacoes) or Estacao do Oriente. The Parque is the river site of Lisbon's Expo '98, and the Estacao is the train station that is about 50 meters from the park. Essentially you will want to go straight at the circle and start going down towards the river, you will see the river very soon after you begin.
These are not city streets, they are divided four lane roads that cut through neighborhoods in a pretty brutal way. But there are sidewalks all along them, and that's definitely where you should be at this point. (I saw a couple of bikes there yesterday)
The name of the street is Av. Almirante Gago Coutinho. When you come to a big Staples store on your left, make a left hand turn and you will be paralleling the river. You can head down to the river when you see twin tall skinny buildings with what looks like a curly ship's prow on top (if you look at my pictures you'll see the buildings I mean).
Once you are on the river, there's a path that continues for 3 km to Sacavem. You mentioned that you were going to ride the N-10, but I've been as far as Santarem now, and I think you'd be much happier and safer just following the Camino, there are many off-road routes, and all are suitable for bikes, at least as far as Santarem. If you look at a map of the stage between Azambuja and Santarem, for instance, you'll see that the national highway goes from Azambuja through Cartaxo to Santarem. But the camino takes you off road, on agricultural tracks through Reguengo, Valada, Porte Mugo, Omnias and into Santarem.
Let us know what you decide to do! Laurie