It is really important that you get your hands on a good university level textbook that explains fuel utilization during exercise.
An important concept in exercise physiology is the respiratory exchange ratio. You see in it literature as R or RER. The respiratory exchange ratio or R tells us what type of fuel is being used during exercise. When we know R we have an idea of what percentage of energy is derived from carbohydrate or fat.
I did not include protein because as you are very well aware protein contributes little to total energy production during exercise.
Here is what you should know…
When R is equal to 1.00 100% of energy for ATP production during exercise is derived from carbohydrates. Fat contributes nothing. When R is equal 0.7% then 100% of the ATP produced is produced using fat as a fuel source. When R is equal to 0.85 approximately 50% of the energy comes from carbohydrates and 50% comes from fat.
You should also know that in order to get accurate respiratory exchange ratio percentages the subject being tested must have reached steady state as far as exercise intensity is concerned.
Really interesting question, I hope I was able to adequately address it, it is something you will have to study indepth but that’s the answer in a nutshell.