KT, what I think you are refering to when you say 'toned' is visible muscle definition. The word "tone" is something that has been used and abused so much by the media that it no longer means what it really should. Muscle definition is more a factor of body fat percentage - you can have some well defined muscles but if they are covered by a layer of fat you're not going to see them. "Toned" refers to how they look in a resting state. To get muscles with definition/size they need to be stressed in order to grow. If those exercises get you to a point where you can no longer do them correctly with proper form, then yes, you will get some benefit from them. The quicker you can get this to happen the better - this is why you lift weights to failure with only a few repetitions. Using light weights and doing endless reps is not how you 'get toned', it's how you build up lactic acid and feel a 'burn'

What you might want to do, KT, is have a look through ExRx's muscle directory and find a few exercises you can do that wont hurt your knees. Deadlifts come to mind, as do calf raises, and sissy squats (my boyfriend with a hyper-extended knee does these).

Bottom line, doing anything is better than nothing, and if those exercises cause you to fatigue your muscles then that's a start - but you may find you grow out of them quickly.

HTH
Nat