I switched to the E-Z Ride stirrups at the same time I got a new saddle. The saddle is a Plantation style and the way the leathers are rigged they turn the stirrup automatically into the correct position for me. My knees are SO much better since that but since I changed three things at once I really can't say what it is. Also, my feet used to go numb after about an hour in the saddle previously and now they never do.
Previously, though, I could avoid the achy knees and numb feet to some extent by mixing some posting trot in with the my ride. As much as I would prefer walk or canter (my guy has a very bouncy trot) the up-down motion was excellent for me. That would be the same as Guilherme's suggestion of walking except I hate trying to find a good place to re-mount on the trail even though I ride a pony and CAN do it.
Make sure the leathers are well conditioned and well turned so the leather itself is not pulling on your feet. Remove the stirrups and put the leathers back together as though the stirrups are in place, oil the leathers well, twist them around, then run a broom handle through them where the neck of the stirrup would be. Leave it for a day or so and they'll be well turned.
Another thing is how the whole saddle fits you- some poorly made saddles hang the leathers at odd angles and forever you will fight to be ok in it: what kind of saddle do you have?
When I do long trail rides in my western saddle I never, ever, have knee pain. I use padded, broad-bottomed stirrups so my foot has a stable/solid resting place. When I've ridden with those gadgets, I hated them. The stirrup leather isn't wrapped around the front of your shin like you're used to, which to me helps me feel more stable on the horse-it's hanging there parallel to his body with my foot stuck in the stirrup= hard to describe but I find the whole set up to be destablizing over rough ground. Also, if you are short, once you add 2" of stuff to manage your stirrup, you may find you cannot shorten the leathers enough to use them
A conclusion is the place where you got tired of thinking. (Steven Wright)
You may want to try wrapping the fender strap below the blevins buckle with a strap like a saddle string, leave string in place, then do a half twist to put buckle parts back together for riding. Do use a hobble strap above the stirrup to keep the fender straps (front and behind stirrup) together.
Doing this twist makes the fender twist by itself, and stay twisted, so there is no pull on your knee when riding. Stirrup stands sideways to the horse barrel at all times, easy to mount with, ride with.
I am seeing a lot of saddles with this treatment on their fenders for Trail Riding, folks who pleasure ride distances on their daily rides. One of the local tack stores will do this to your saddle for a small fee. You could wrap the string on the fender yourself too, for making the twist work on your saddle.
Here is a photo at Option 3, of the wrapping and twisting method.
Oxbow stirrups. Yes, I doubted, but tried, and no knee pain even all day, even on rugged mountain trails. Seemed counter-intuitive to me but there you are. When riding English, I use the four way flex stirrups and likewise, no more knee (or hip) pain.
...all suggestions are great, also lengthen your stirrup a bit. You should have a nice long leg, just tip your toe up into your stirrup, with a very slight bend in the knee. This will also help prevent you from tipping forward or gripping with your knees.