MayBee - walking in the countryside was one of my top pleasures as well, and I have had to curtail it greatly (I'd add that I suspect steep hills / cliffs are the worst)
I will share what I do to stay positive, as it may help:
I do water-based exercises about 3 x a week, which do me a lot of good and help to keep the pain under control. I also do pilates at home, using a board & yoga-mat on the spare bed to save me getting on the floor! This all keeps my muscles working.
I go out for a walk of about 1-2 miles when I am not in too much pain. I use nordic poles, and I mostly go out in company. This does me no good physically, but helps mentally.
Sometimes we go to a country park and I hire a scooter so DH can walk alongside me and we can go further.
I look at a beautiful picture DH took of me & the children on top of a mountain, and I think 'I am so lucky to have done that in my life'.
I have known a few people who have 'refused to give up' and kept walking through a lot of pain - their decision of course, but it has, I think, hastened their physical (and therefore eventually mental) decline. So I think finding a regime that suits you is important.
I hold you in my thoughts.