“This change in the fuel-burned ratio is why you might hit ‘The Wall’ when trying to run a marathon as fast as possible, but you might not during an ultramarathon. A marathon run at faster than your normal training pace can use up all of the glycogen stored in your muscles. When that happens—usually in the final 10K—your muscles turn to fat to fuel your stubborn insistence on reaching the finish line. But burning fat requires a lot more oxygen than burning carbohydrate, so to meet that demand for more oxygen, you have to slow considerably, usually by a minute per mile or more.”

Full article

https://www.runnersworld.com/uk/health/weight-loss/a32834002/facts-about-fat-burning-and-running/