Ahh, Hugo.. I sympathise with you in the 'knees' department.
Us humans are badly designed and it always seems to be the knees that pack in first as we grow older.
Like you I had to pack in football about 3 years ago, as my knees were in pain during and after every match.
It was no great loss to the game, although I played thousands of times I was never actually any good.
In terms of walking, as arthritis takes its toll, its always the downhill walk that causes pain because of the constant impact going down through the joint.
Uphill walking is fine. well.....its fine for now.