Most carbohydrates, especially starches, are in the form of chains of glucose molecules. Those molecules are cleaved in the digestive tract, and the glucose enters your bloodstream. Since the presence of glucose above a certain level is dangerous, and possibly even life-threatening, the pancreas, when it senses this high load of glucose in the blood, secretes insulin to get it out of there.
A high level of insulin in the blood does two things: it causes the muscles to take in glucose and burn it instead of fat (storing any extra as glycogen), and it causes the adipose tissue to convert the glucose into trigylcerides for storage. These two actions get the glucose out of the bloodstream and avert the danger.
If your muscles are used to running on glucose exclusively, it takes many weeks for them to convert over to burning ketones and fatty acids. The fat cells, as I understand it, pretty quickly regain their ability to break down triglycerides and release the fatty acids to be metabolized. One of the hallmarks of metabolic flexibility, which is a property of fat-adaptation, is the muscles’ ability to switch between burning glucose and burning ketones and fatty acids. In this context, note two things: the insulin level must subside before fat metabolism becomes possible again, and excess glycogen stored in the muscle cannot be released—it must be turned back into glucose and metabolized right there in the muscle.
Since the body normally has only about a tablespoon of glucose circulating in the blood, you can imagine that carbohydrate in any quantity can occupy the metabolism for some time.
