What are you doing for cardio? I bet you need to slow down and keep your heart rate down. When your heart rate increases, your body turns your exercise into an anaerobic exercise and does not have enough oxygen to burn fat. When this happens, the body will start to burn glycogen that is stored in the liver for instances like this. Glycogen is a simple storage of sugar molecules used in "emergency" situations. You dont want to get into your glycogen storage. You can have have a few hundred calories (400ish) of stored glycogen. Thats a lot of calories to burn before your body will start burning fat. Try a slower paced exercise like a stair mill, or walking briskly on the treadmill. I try to keep my heart rate at 135-145bpm, so I walk at 3.5mph at a 3.0 incline.
I agree with just about everything else in your post except for the "fat burning zone" part, there have been a lot of articles written debunking the fat burning zone.
First, it's true, you burn a higher percentage
of fat than carbs while working out at low intensity. However, those carbs you don't burn will simply be turned into fat at a later point in time. Point being, fat and carbs are essentially the same pot of caloric energy when it comes down to it, they are just at different states in the body. You need to focus on total caloric output, not if you are burning fat vs. carb calories.
Second, the total amount
of fat burned (over the same amount of time) will be almost the same in low vs. high intensity workouts, with much more total calories being burned in the high intensity workout. Low intensity workouts can take up to double the amount of time to burn the same amount of calories as a moderate to high intensity workout.
Third, aerobic training does very little to nothing to preserve the muscle. Yes, this can be offset by weight training, but 1lb of muscle can burn somewhere in the realm of 50 calories a day just by existing and maintaining, so why not do all we can to preserve what we/he has?
I'm not saying you need to end your workout barely being able to scrape yourself off the treadmill, but you need to be doing a little more than walking. There's no great, ripped athlete out there that got to that single digit body fat % by walking quickly.
Some articles that may have worded it better than me: