Leslie:
Hi there.
I think I would be tempted to consolidate the increased strength and endurance of my knees at this time, rather than incease their load. Maybe give it another month to strengthen more, then add in another day's cardio, and perhaps make it Kick Punch and Crunch, which is mainly low impact, but a high cardio intensity punch, rather than more explosive step or hi-lo.
If you are doing weight training for each body part twice a week, that is all you need. To increase the fat loss on the butt, just take the weights to your max, gradually. The more muscle you build there, the more you will burn of all-over bodyfat. Looks like Legs and Glutes can tone and shape that area for you, but the PS series and Slow & Heavy and PLB would do a better job of actually building the glutes, I believe.
If you are craving another cardio boost, the previous poster's suggestion of a swimming day or even two, is excellent. It will also provide another workout for the back and shoulders and abs, with no impact. Swimming gives wash board abs. Cross training is key to over all fitness.
Or you might try having a walking day, which will still burn calories for you and tone those leg and glute muscles, but with much less impact. And if you live in a hilly area, this is ideal. You could do walking fartlek/intervals, short intensity bursts of one minute, followed by thre minutes at a slower recovery rate, and repeat as much as you like. Going up hills will also provide an extra cardio challenge. With this type of workout you are doing your knees a favour with less impact, but the interval training vastly improves your cardio endurance and burns more total calories than a steady state walk around the park. If you have a friend who wants some fresh air too, this would be ideal because you could challenge eachother up the next hill, speed walk to the next fire hydrant or old man sitting on a bench, whatever!
And finally, it has been my experience that sometimes less is more. If your body is well rested, it will perform better for you when you really want to punch up the intensity.
And really lastly, knees are delictae pieces of human machinery, we shouldn't mess with them! I am in awe of the human body!
See ya, hope some of this works and appeals to you
Clare