I've been getting into hiking for the last half a year and can hike up to 25km somewhat comfortably. I have been toying with the idea of doing thru-hikes or some of the different St. James Ways in Spain in the future but have never yet done long hikes in succesive days.

I have been lurking here for long and looked on the internet. I know blisters and the psychological aspect of watching unchanging landscape for days are among the worst enemies (save wilderness hazards of course).
Right now I'm mainly interested in the physical aspect of this and I'd like to know if you can recommend me some kind of training plan previous to a big trip like this.
All advice I have read boils down to "just walk" the previous month. Go out every day and increase length and amount of slopes with time. Also test boots on this walks and carry the weight you plan on bringing.
I can get my head around this, but it's too broad.
Can someone here give some personal opinion on training specifics?
As context I don't plan on going longer than 5 days for a first trip that wouldn't have daily trecks longer than 25km. But if training changes significantly depending on max treck length, total trip time or amount of accumulated height I would like to know that as well.