Hello Pathfinder:
I cycled the Caminho Portuguese from Lisbon to Santiago in September 2015. I rode both the Caminho and took side roads parallel to it while in Portugal. At Porto I took the inland route to Valenca/Tui. I decided to cycle as opposed to walking as I wanted to cover more ground, specifically to have time to sight see.
Based on my experience, I would recommend not to camp out. I did not see many opportunities to do so on the Caminho. Portugal is very hilly, especially after leaving the Tejo river plain. BTW, the first 15 kms out of Lisbon are rather industrialized and very uninteresting. Take the train to Azambuja and start walking from there. There is a very good albergue (difficult to find though) in Santarem called Hostel Santarem run by Mario Gracias. After Sanatrem, you will walk to Golega, a great little town and home to Portugal's only Nobel literature winner, Jose Saramago. If you like horses, Golega is the horse capital of Portugal.
When you reach Tomar there is another great hostel, Hostel 2300 managed by Eutimo (Tim) Sousa. Tomar is really worth exploring. From Tomar, take the bus to Fatima. If you walk it be prepared for a hilly and very uninteresting walk. After walking to Fatima, (if you elect to do so), you will marvel at how much faith the Portuguese have. Fatima and its location (middle of nowhere at the time of the apparitions) stun the senses.
The route to Fatima is well marked with blue arrows. The yellow arrows you encounter will be for the Caminho. Be aware that the Caminho (yellow) arrows are sporadic between Lisbon and Porto. You have to have a good sense of direction and anticipation. The soil is sandy and at time challenging to walk. You will also be introduced to the national tree of Portugal: the eucalyptus. It is grown in large tracts for use in the paper factories.
Again, I recommend that you drop as much weight as possible for your walk. Albergues/hostels/hotels are cheap in Portugal. Food is excellent especially if you like fish. As you move north above Porto and if you take the inland route, the diet changes to meat and pork.
Do not miss Coimbra, Portugal's first capital and home to one of Europe's oldest universities. It is on the route.
Feel free to ask more questions.