You're probably one of those people who just genetically finds things like kicking and punching to be easier or more natural. I know my boyfriend, even after not working out for ages, can do karate and kenpo kicks and punches without breathig very heavy. I sweat a lot during Kickbox, but I don't get exhausted and breathless. I just find step and hi/lo to be more natural to me.
I did, one day when I was feeling really energetic, use the intervals at the end of Step n Intervals after the Kickbox cardio portion to give me a good kick in the butt. Went out with a bang, as it were. That helped. But I think the idea with the design of CTX is that the intensity varies, so consider that your lighter intensity day. If you did cardio for the same duration and intesity every day, your body would adjust pretty quickly and you wouldn't see such great results. I've also repeated sections when I wanted a little longer of a workout, but I do love the way Kickbox is laid out. Great moves and music.