I found an article in a exercise magazine recently that said you need to work out at least 30 minutes to allow your muscles to burn the sugar that's present locally before your body starts to burn fat. It said that it was biologically simpler for your muscles to convert the sugars in your body before doing the hard work of breaking down the fat into simple sugars for use. So, I'd say that there's not fat burning range nor cardio range.
I found another article that really helped me.
To figure out your heart rate min an max while exercising:
220 - (your age) = Maximum Heart Rate (MHR)
Figure out your resting heart rate for a minute (RHR)
Your Heart Rate Reserve (HHR) is the MHR - RHR = HHR
Now, your minimum target heart rate is:
(HHR x 0.65) + RHR = Minimum
Now, your maximum target heart rate is:
(HHR x 0.85) + RHR = Maximum
Mine worked out to be a 166 maximum and I find that when I'm close to that number I'm pretty much out of breath so I think it's a pretty accurate way of exercising.
*add in heart rate after the word "elevated" in the first paragraph.
I'm afraid I won't be of much help. I've also heard conflicting points on this. On the one hand, adding intensity can increase the calories burned. On the other, if you get into that infamouse "cardio" level with an elevated, then you're just burning carbohydrates and doing nothing of use.
Those are the two schools of thought.
What I'm doing right now is mixing the two. I have one day of intense cardio, where I work to increase my heart rate and build up aerobic endurance. Then I have an active rest day, usually following a day of intense strength-training, where every muscles is sore already. On that day, I try to stay within the fat burn zone with my heart rate.
Honestly, I've noticed no great difference between the two, except the higher intensity workouts seem to do more good as far as building endurance. And they're more fun, because they're a challenge.
To lose fat--hence, tone up--you will likely need to focus on your diet. Aim for a high protein, high fiber, and low carb diet. Because you are already very fit, if you intend to cut calories, you have to be careful how much you cut. Do not go overboard here. You only need a slight calorie deficit to make a difference (250 a day at most).
Also consider that sometimes the "fat" we think we have is actually just water. Fat has a very specific feel, like the cellulite we all know and love. Sometimes puffiness is merely water retention. If it's jiggly and sloshy, it may be water, not fat. Try increasing your water intake and decreasing sodium. That may help.