The most important factor for adapting the aerobic energy system to exercise is the ability of the cardiovascular system to pump oxygenated blood to muscles.
This capability ensures that the working muscles receive sufficient oxygen, which is essential for effective aerobic metabolism and producing ATP through oxidative phosphorylation. Improved cardiovascular function enhances endurance performance and promotes the efficient utilization of fat and carbohydrates for energy during prolonged exercise.
While the other factors mentioned (transforming ADP to ATP and the reliance on the lactic acid system) are important, they are secondary to the primary role of oxygen delivery in aerobic adaptations.