The immediate thought that comes to mind when you bring up conditioning is that this is an area where you better err on the side of caution. In general you can not "harden" parts of your body without consequences. Apart from the arthritis pains themselves, on the combative side and further down the road, an old cripple is an easier target than an old but otherwise healthy person. What you can do is thoughen up soft tissue and desensitize the nerves in the area by doing things like working on bags and slapping the area with your hands. That should work well without adverse side effects. But in general if you can't do something with your body as it is, it might be better to shitcan that and find something that works right now. Just some thoughts on conditioning.
Regarding the knee, you probably didn't do that knee right. Hitting with the kneecap is as I understand it a big no-no. (It's just a loose piece of bone held in place by tendons and stuff) Better to contact with the top. so if you're going to knee something, it better be no higher than your waist area and well within your reach.
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