Which is why I was suggesting the glutamine. It is only in theory, and I am no scientist (that is for sure), but some research has shown that glutamine can
replenish glycogen stores. So, you could have some whey to give your body some amino acids (so your body wouldn't tap into your muscle during the cardio) and a hefty dose of glutamine. The insulin response from just protein should be pretty low, thus making it easier to tap into your fat stores during cardio. Maybe.

Not the best option, but far better than exercising after a multi-hour fast.