Answer:
I believe the amount of mitochondria in our body only increases after long term exercising.
When you are exercising, you do not produce more mitochondria straight away. Even after you stop, the amount of mitochondria will be the same.
I believe the production of mitochondria and the improvement of your muscles is the same. You need to exercise for awhile before you can get more mitochondria in your body.
As for your amount of food needed, it is not according to the amount of mitochondria you have. It is according to how much energy you need.
You should be able to tell how much food you need. So, you actually wont produce more fats. As long as you eat the correct and moderate amount of food.
You will not produce more fat when you stop exercising than when you never exercise. Like I said, the amount of fat produced is according to your food intake. Your food intake is according to your energy needs. As long as you eat moderately, you will not produce more fat. In fact, you will be able to burn fats better after long term exercise because your body will be healthier and able to moderate itself.