Physiological changes in response to training

According to published research, an increase in training, where the heart pumps against a greater resistance, leads to increased cardiac demand, and a rise in blood pressure, heart rate, blood flow to muscle, and oxygen consumption, to meet the increased metabolic demands of exercise. Over time, aerobic training has been shown to improve the lactate threshold and V̇O_2max, with the size of the effect moderated by both volume and intensity of exercise.

An individual’s maximal oxygen uptake (V̇O_2peak) improvement represents an increase in cardiorespiratory fitness, whilst an improved running economy, is likely to the result of an increased ability to move more economically, and reduced energy demands, suggests a higher likelihood of success in an endurance event 

Kenney, W. L., Wilmore, J. H., & Costill, D. L. (2015). Physiology of sport and exercise. Champaign, IL: Human Kinetics.

McArdle, W. D., Katch, F. I., & Katch, V. L. (2015). Exercise physiology: nutrition, energy, and human performance. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.

Rønnestad, B. R., & Mujika, I. (2013). Optimizing strength training for running and cycling endurance performance: A review. Scandinavian Journal of Medicine & Science in Sports, 24(4),603-612. 

Zaryski, C., & Smith, D. J. (2005). Training Principles and Issues for Ultra-endurance Athletes. Current Sports Medicine Reports, 4(3), 165-170.