Q: is the fat-burning zone vs. cardio zone benefits right? i e., targeting fat when in the fat burning zone and carbs in the cardio zone...

Or is this just a myth?

A: For the most part it's a myth. Check out http://www.mikesfitness.com/content/counting-calories-burned-on-treadmill for a related discussion. There will be some difference on calories burned and on your metabolism from different types of cardio but its nothing as cut-and-dry as the 'zones' want to make you believe.