It's like you said "what Ranger does not have trouble with his knees?"
My knees are toast too, (no knee pads were ever used in my era, we just took a knee in rocks / any terrain.
Firstly, physical therapy should help. I think flutter kicks help,
also swimming helps and the ol' mountain bike.
It's the pounding from running and the twisting stress from rucking/ hiking that agravates the knees.
Somedays after a run, simply walking down a flight of stairs is too much for me!
Don't ignore the pain Ranger, in your line of work you need to be mobile.
I'm no Doctor, just another Ranger with fucked up knees
Good Luck Bro'.