What structural changes in the muscle of the endurance trained athlete help with producing ATP aerobically (using oxygen)

What structural changes in the muscle of the endurance trained athlete help with producing ATP aerobically (using oxygen)?

2. Explain why after doing 2 months of endurance training if you do the same submaximal activity (eg. run at 5-7mph) your heart rate and the lactate concentration in your blood are lower than before training.