I think that Javier knows a lot about this route, so maybe he will jump in here. My concern would be the marking. I was just in Lisbon last week, and spent a fair amount of time in the area around the cathedral. I never saw (but wasn't looking for) any markings of the route. And my eyes are fairly sensitive to arrows! The online guide that Sil refers to has detailed walking instructions ("keep to the left on the Calcada da Cruz, turn right onto Madre de Deus," etc), so it may be that they are a substitute for arrows.
I was in a church south of Lisbon on this trip, in a town called Santiago de Cacem, and appropriately the church is also the church of Santiago. There are some wonderful old carvings, and a stone relief of Santiago Matamoros. I asked the man at the entrance about the Caminho in this part of the country and he told me it was essentially ignored. He said there have been noises made about reconstituting the trail from the Algarve (southern coast) but no action.
My impression was that the Camino Portugues is marked north of Porto only, but this information could be out of date. When I was considering walking it in 2004, we ultimately decided to start in Valenca do Minho, right on the border with Spain, because our information was that there was not much marking or infrastructure in Portugal. I do know that there have been several albergues opened in Portugal since that time, but I believe they are north of Porto as well. But I think you will need to be prepared to stay in private accommodations for most of the walk.
I don't mean to be discouraging, and as I've said, it's very possible that my information is way out of date. I think it would be a wonderful walk, from Lisbon north, and I hope that if you undertake it, you'll let us know how it goes! Good luck, Laurie
