Just a note, when doing any aerobics workout you should be able to say 3 words for every breath, it should be a bit of a challenge to get those 3 words out, but do able. If your panting so much that you can't, you're working your heart and lungs too hard and can cause damage. So you always want to gear your workouts to be able to repeat 3 words of whatever Cathe says. There your working some where between 75-85% of your max heart rate. Which is great for fat burning and burning calories. I know we all can do harder workouts then this, but the cost of doing them is quite high if we do them again and again. And you actually lose some fat burning ability if you do a workout over your 85% max heart rate. If you don't want to waste money, you can always buy a harder tape and modify it so your in the right range, and then do the harder moves when your body says okay this isn't tough anymore.
HTH,
Kit