Ok a very stupid question, I can't wrap my head around this. The sentence :
We have been walking all over the countryside since last weekend. --- Is it a passive construction?

If it is then firstly, why?
Secondly, how would you improve the sentence.

many thanks

this is "present perfect progressive" (there's a mouthful)

It indicates both present perfect: something that started in the past and is still going on right now in the present, and progressive: a state of action that is continuing or in progress.

The sentence is technically fine as is - if that's what you're trying to say, it's fine.