Re: Bad knees
This will probably start a fight, but the best single thing you can do to strenghten the knees is the weighted Back Squat. Done properly, it is also the single best exercise to prevent osteoperosis and back injury. Done improperly, it can be the worst enemy of all the above, so you must do them properly. How to productively add them to your overall routine if you are unwilling to reduce Aikido training loads during strength building can also be an issue.
Suwariwaza and hamni handachi are by far the worst Aikido activities for the knees. It involves twisting and pressures on the knee that are much more likely to damage it than what is involved with falling or throwing. Anything you can do to spend less time knee-walking will probably help increase your knees' useful life.