Knee Pain, but Only When Walking

I’m hoping for some advice here, as I really have no idea what’s going on.

Recently (last 6 weeks or so I think) I have been getting this rather bad pain in my left knee when I go for walks. It’s fine in the beginning, and then usually after 10-15 minutes my left knee starts hurting and gets progressively worse, even though I’m walking at a moderate comfortable pace out in the woods. Several times it’s gotten to the point that even my left shin and hip have started hurting as well, and a few times it’s gotten to the point where I’ve just had to stop and take a break for a few minutes. After finishing a walk the pain usually goes away completely in maybe 30 minutes up to an hour.

The really weird thing is that it’s only while walking; I have no pain whatsoever while squatting or deadlifting or any other kind of training, doing box jumps, jumping on a trampoline, climbing a rope, nothing else. Just walking.

This has really come out of the blue for me, as I’ve been used to walking between 1 and 2 hours almost every day my entire adult life, and I can’t really make sense of it. Any ideas or advice is greatly appreciated.