I see some time has passed since this topic was opened, hope someone still sees this.
I also like hiking in hills, not such long paths (under 1000m elevation), but still considering I go 1-2 times a week and at pace, after some time I had similar issues with left knee, but only downhill (true I run).
Since the problem was getting more serious, I paid more attention to walking technique. First I noticed I didn't have even steps, loaded left leg more than right and that helped partially. Then I realized if I have bent knees and lean back a bit more, I really relieve knees, true thigh muscles are more loaded. But better muscles loaded than knees, and over time you strengthen muscles and both problems disappear. And so now I can go downhill without issue, without any pain, even running.