The body is a delicate and plastic instrument, which responds readily to the thoughts by which it is impressed, and habits of thought will produce their own effects, good or bad, upon it.”
Your body is adaptable, and your repeated thoughts matter more than a single thought.
That’s a strong connection to fitness because it’s almost the mental equivalent of training. One workout doesn’t build your body. Repeated workouts do. One meal doesn’t determine your nutrition. Your eating habits do. And one negative thought doesn’t define your mindset but repeatedly thinking the same way can develop into a mental habit.