As I get older, 66 now. I find that the muscles involved in casting a Spey rod are only kept in shape by fishing. I exercise and treead mill but only fishing works the muscles that you need to cast.
I have arthritis and a back that was broken back 1968. Those things you just have to learn to deal with. Most of us as we have grown older have pain of some kind to deal with. Some Advil befpr leaving home helps, and sometime you just have to concentrate on something else and ignore the pain.