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.