Ola' Liana -
In my experience, late July/August can be very humid/muggy and hot, plus mosquitos. Plus August is the portuguese vacation month - i.e. it's going to busy on the road, beaches, restaurants, hotels...
I would opt for good walking shoes - portugal has lots of cobblestone street, hard walking ... i wouldn't dream of walking long distances with sneakers. but you know feet better ... still, proceed w/ caution in that department.
you can check
www.wunderground.com for Porto and other northern places ... and in the Historic section of the website do check for the month of july/august in the previous years. might give you an idea of DEW point and Temps.
furthermore - in the subforum for the portuguese camino you find excellent answers of multiple questions/concerns/inquires.
best wishes and bom caminho!