Strange knee pain

I have developed a situation where I cannot walk very far without pain in the outside of my knees, but yet can ride 100km and not feel a thing. I only noticed this a few weeks ago when doing a bushwalk; about 6km into it, the outside of my knees was so painful I could barely walk. It was particularly bad when walking downhill. Both knees became sore at the same time. It took about two days before I could walk normally again, but in the meantime I got on a bike and found I could ride it perfectly well with no pain like nothing was wrong. My guess is that something has happened as a result of not walking as much as I ride (some kind of change in muscle development?). The same thing has happened each time I have gone for a walk of more than a couple of km. I certainly have been riding much more than walking over the last six months. There's plenty of advice about knee pain from cycling but I have it the other way round. It is a strange situation when someone can ride all day long but can't walk more than a few blocks.

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1995 Trek Y22
1996 Trek Y33
1997 Trek Y5
1998 Trek Y33
1998 Trek Y22
1998 Trek Y11
1999 Trek Y Superlite 200
2006 Trek 3900

Walking uneven surfaces only one way can cause knee problems. and that bush walk would have put your knees and all leg muscles in ways they wernt use to. Maybe you legs are use to firing in a way suited to cycling. Maybe your legs needed a rest that day. At the 6km point walking down hill would have put some stress on them that they wernt use to.