Fernman, I can wholeheartedly recommend the regular use of a cross-trainer to strengthen the legs, whereupon you should be able to leave the poles behind. Worked for me - I no longer have a knee issue, even on steep descents.
A cross-trainer is....? Please tell me, for I don't know.
But it's not a knee issue with me, my problem is age - no, I'm not going to reveal it - so my entire legs ache and tire, including all the joints, but most particularly my lower legs between knees and ankles. I've never had much in the way of muscles, but I used to come home from backpacking trips with muscles on muscles. Now they are just wasting away.
After 7.25 miles yesterday, most of it reasonably level until towards the end when there was a long steep descent of the Chiltern escarpment and, after a level stretch, a cardiac arrest climb, I was absolutely whacked for the rest of the evening, I even went to bed early (having a mild cold doesn't help). But I more or less recovered after 9 hours sleep!
My speed, when I worked it out later, was 2.25 mph.
And the problem with backpacking, I found long ago, is that you get cream crackered in one day, when you walk for more hours than you usually do on a day walk, carrying a much bigger load too, then you've got to do it all over again the next day, and the day after that, and so on.