Final answer:
Debi's painless marathon experience can be attributed to the release of endorphins, her body's natural painkillers, and her likely higher proportion of slow-twitch muscle fibers that are better suited for endurance running.
Step-by-step explanation:
Debi's lack of pain during her marathon run was most likely due to the release of endorphins, which are natural pain-relievers produced by the body. Endorphins are often released during vigorous exercises such as running, and can contribute to a sensation known as a "runner's high." This is a state of euphoria combined with reduced anxiety and a lessened ability to feel pain. Additionally, endorphins may also help regulate mood, contributing to the feeling of well-being that can accompany intense, sustained exercise like marathon running.
A marathon runner's body adapts to the demands of long-distance running by having a higher proportion of slow-twitch muscle fibers, which are more efficient for endurance activities. These muscle fibers help athletes like Debi sustain activity over long periods without succumbing to fatigue as quickly as they might with a higher proportion of fast-twitch fibers. The combination of physiological adaptations and the biochemical effects of endorphin release likely contributed to Debi's ability to run a marathon in less than four hours without experiencing pain.