The Fatima groups with the help of local authorities waymarked a route from Estroil to Fatima on the 29 April this year. From Sintra it is basically the old camino route from Lisbon you can see on most credencial maps and then very near to Fatima it deviates from the old route so it can go to its new destination, It is called Caminho do Mar and it is marked with both yellow and blue arrows.From what i have seen written of it they have tried to take it off road as much as possible, it had been planned for a while then all the people involved waymarked their areas on the same day. Not sure how close it actually comes to the coast and there is very little central information as to distances etc.
This route may be of some use to you on some stretches of your pilgrimage.
Good Luck and Bom Caminho.