When an untrained person begins endurance training it is improvements in stroke volume that develop in parallel to aerobic capacity. 

Pulmonary diffusion and maximum heart rate do not appear to change considerably with training and are more related to age and genetics.

After regular training and when the heart’s stroke volume is close to its maximum potential, research suggests that cardiac output does not change much.

One year of prolonged and intensive endurance training leads to heart adaptations similar to those observed in elite endurance athletes. This study suggests that long-term, intense exercise training can lead to significant changes in the heart, including increases in cardiac muscle mass similar to those observed in endurance athletes. The study also found that the right ventricular mass and volume increases early in an endurance training program and may be necessary for the eventual increase in left ventricular volumes seen in endurance athletes.

However, the study also highlights that even a year of training in a previously sedentary individual may not be sufficient to achieve the same level of VO2max and cardiac compliance/enlargement as seen in successful endurance athletes. It is important for clinicians to understand not only the extent of cardiac plasticity but also the dose-response nature of this adaptation, as lower doses of training primarily lead to increases in left ventricular mass without increases in volume, while longer durations or higher intensities of training are needed to increase volume.

Pumping large volumes of oxygenated blood via a high cardiac output is critical to increasing V̇O2 max, therefore training should target improvements in this area. Achieving heart rates close to maximal levels will do this and facilitate adaptations via cardiac remodelling. This includes improvement in the volume and thickness of the left ventricle— the heart's biggest pumping muscle. Greater stretch from this increased volume also results in greater elastic recoil. by stretching the heart muscle and filling it with a lot of blood increases its contractile strength to deliver more blood with each beat.

Endothelial protection against cardiovascular disease 

Aerobic exercise has been shown to have a positive effect on endothelial function, which is an important risk factor for cardiovascular disease. This is because aerobic exercise increases the production of endothelial nitric oxide (eNO), a molecule that plays a crucial role in regulating blood vessel tone. Regular aerobic exercise has been shown to activate the enzyme endothelial nitric oxide synthase (eNOS), which results in increased eNO production and improved blood flow. Additionally, regular aerobic exercise has been shown to reduce the degradation of eNO into reactive oxygen and nitrogen species (ROS and RNS), which can further improve endothelial function.

Another benefit of aerobic exercise on endothelial health is the stimulation of macrophage-mediated reverse cholesterol transport. This occurs through the activation of a protein called peroxisome proliferator-activated receptor gamma (PPARc), which is involved in the regulation of lipid metabolism. By activating PPARc, aerobic exercise probably promotes the removal of cholesterol from the bloodstream and reduces the risk of cardiovascular disease.

The autonomic nervous system (ANS) plays an important role in regulating the body's cardiovascular system, including blood pressure and heart rate. As we age, the ANS can become dysfunctional, which can lead to an increased risk of cardiovascular disease. Endothelial dysfunction, which is a common feature of aging and cardiovascular disease, also contributes to this increased risk.

Zone 2 training has been shown to have a beneficial effect on the aging autonomic nervous system and preventing baroreflex dysfunction. The baroreflex is a mechanism that regulates blood pressure homeostasis by sensing changes in blood pressure and adjusting the activity of the sympathetic and parasympathetic nervous system accordingly.

Aerobic exercise training has also been shown to increase heart rate variability (HRV), which is a marker of autonomic function and a powerful predictor of cardiovascular disease outcome. HRV increases with aerobic exercise training in older people. 

The beneficial effects of exercise on HRV have been attributed to reduced activity of the hormone angiotensin II, increased production of nitric oxide (NO) and improved balance between the sympathetic and parasympathetic nervous system. An increased HRV is associated with a better prognosis.

In summary, an aging and dysfunctional autonomic nervous system and endothelial dysfunction have a synergistic effect in increasing the risk of cardiovascular disease. Zone 2 training and aerobic exercise training have been shown to have a beneficial effect on the aging autonomic system and prevent baroreflex dysfunction. Additionally, aerobic exercise training increases heart rate variability, a marker of autonomic function, which is associated with a better prognosis for cardiovascular disease.