Hi, Maria, I can't help with details on walking in Portugal, but I would strongly recommend reservations in Fatima in August.
The 15th of the months from May through October are always highly visited periods at the shrine, as these are the anniversary dates of the apparitions. Tens of thousands converge on the plaza on these days in any given year. This year that's expected to be far more (with hundreds of thousands expected this weekend).
Additionally, August 15th is an important "holy day" in the Catholic church in honor of Mary, making it even more attractive to Marion pilgrims. So add that into the equation as well.
Start checking availability now and see how things look. If you're not going for religious reasons, avoiding the middle of the month might make finding a bed a bit easier.
Good luck and happy planning!