hi,
@Octosaurus, welcome to the forum.
From Lisbon, you have three choices (at least). The so-called coastal route, which really doesn’t spend much time on the coast at all, and goes through Sintra. We have a sub-forum on that route
here. There aren’t many members who have walked it, but there are almost 60 threads, so that should give you some good info. Do not expect to see anyone else if you walk that route, though!
There is also the Trilho das Areias, (the path of sand) which does follow the coast. We don’t have a sub-forum for it because there is just not much traffic, but if you go to
this post, you’ll get links to a few web sources.
And then of course, the non-coastal route, the Central route, which goes along the Rio Tejo for a while. Lots of info on that option.
This is a start, maybe others have more info! Bom caminho.