Your body needs to fuel any activity you do. It will use a combination of whatever it has and whatever fuel will work - food you have recently eaten, glucose already in your blood, liver glycogen, stored fat, muscle glycogen, and in extreme cases, protein.
Your body needs insulin for the first 3 fuel sources I mentioned above - food you have recently eaten, glucose already in your blood, and liver glycogen.
In your example, if you do a somewhat strenuous exercise like that without breakfast and without insulin, your body will release liver glycogen for fuel. That 200 is the result.
There are ways to make this better. You can eat first and have a little bit of insulin to provide fuel. You can take insulin to counter the liver glycogen, but you would need to make sure you are going to get the liver glycogen response, and that is somewhat risky without a lot of practice and with the Metformin, as this reduces your liver glycogen response. Or you can reduce the intensity a little bit, and make it a lower effort, which would allow your body to use some of the other fuel sources available, like more fat.
Are you on a low-carb diet?