Carbohydrates are the only fuel that can be broken down and used anaerobically.
It takes anywhere from 30-60 minutes for fat to become available to the muscles as fuel in the form of free fatty acids, so during prolonged exercise, once the adaptation period to the exercise bout is over, less carbohydrate is used and more fat is used to supply ATP for the working muscle. It is not a comprehensive medical review and does not include all the potential medical conditions, issues, or considerations.