The evidence suggesting regular aerobic exercise can reduce the development of some types of cancer is solid. Perhaps even stronger is the evidence suggesting regular aerobic exercise can have substantial impacts on cardiovascular health. With regular aerobic exercise, the heart's efficiency increases, producing many beneficial effects for the body. Improved heart efficiency can also lead to a reduced risk of developing heart disease.
Improved Heart Efficiency
Heart efficiency and cardiovascular fitness can be determined by several different methods. One method is to measure an individual's resting heart rate. A lower resting heart rate after a period of aerobic exercise training indicates improved heart efficiency. The stroke volume (amount of blood ejected by the heart each beat) is increased and this essentially means the heart muscle does not have to pump as many times to get sufficient blood to the body. With aerobic exercise training, the heart muscle is strengthened and it does not have to work as hard as it did before the aerobic exercise training.
For example, at rest, an untrained individual may have a heart rate of 70 beats per minute (bpm) and a stroke volume of 71 ml / beat. This is equal to a cardiac output of approximately 5000 ml / minute. On the other hand, an aerobically trained individual with the same cardiac output will have a resting heart rate of 50 bpm and a stroke volume of 100 ml / beat.  The trained individual is able to get enough blood to his body with fewer beats than the untrained individual. This demonstrates just one adaptation the body goes through with regular aerobic exercise training.
Other adaptations that accompany regular aerobic exercise training include an increased total blood volume, increased blood flow to active muscle, and an increase in mitochondrial size and density. This increase in mitochondrial density is important for the use of fat as fuel during sub-maximal aerobic exercise. When a fat molecule is to be broken down to be used as fuel, it is broken down into a fatty acid and a glycerol molecule. The fatty acid is transported by the blood to the mitochondria where it is processed to be used as fuel. With an increase in the number of mitochondria in the muscle cells, the body can process and use more fatty acids as fuel. This preserves the potential energy stored as carbohydrates for other uses.  In other words, the body can use fat more efficiently during aerobic exercise and save the more precious carbohydrate fuel.
Aerobic Exercise and Heart Disease
Regular aerobic exercise can also reduce an individual's lifetime risk of developing heart disease. A study done at Stanford University found that the best predictor of death is lack of fitness.  Another study done also found that "physical inactivity is an important risk factor for cardiovascular disease."  How fit an individual is plays a major role in determining how long a lifespan that individual may have. This improvement in fitness can be achieved through regular aerobic exercise.
Another way that regular aerobic exercise can reduce the development of heart disease is through an increased number of capillaries in the body. When a person is aerobically trained, more capillaries develop to improve the oxygen and carbon dioxide exchange between blood and cells. If an artery is damaged or if blood flow is blocked, the blood can easily be rerouted to where it needs to get to and deliver the necessary oxygen. This increased number of capillaries occurs not only in the heart, but also in the brain and all other parts of the body, so also reducing the risk of stroke. 
Studies have also shown that regular aerobic exercise may be able to reverse the effects of heart disease already occurring in an individual. Of all the risk factors associated with heart disease, many of them are modifiable by regular exercise and weight control. Risk factors for heart disease which can be reduced or eliminated in an individual through regular aerobic exercise include hypertension, high levels of fat in blood, type 2 diabetes, obesity and a sedentary lifestyle. Through regular exercise, these conditions can improve, and as a result, heart disease can be prevented and in some cases reversed. 
 William D. McArdle, Frank I. Katch and Victor L. Katch, Exercise Physiology: Energy, Nutrition, and Human Performance, 6th ed (Baltimore: Lippincott Williams and Wilkins, 2007): 354.
 Ibid., 478-80.
 Steven G. Aldana, The Culprit and the Cure: Why Lifestyle is the Culprit Behind America's Poor Health and How Transforming That Lifestyle Can be the Cure (Mapleton, UT: Maple Mountain Press, 2005), 142.
 James B. Carter, Eric W. Banister and Andrew P. Blaber, "Effect of Endurance Exercise on Autonomic Control of Heart Rate," Sports Medicine 33 (2003): 42.
 Aldana, 143.
 Philip E. Allsen, Joyce M. Harrison and Barbara Vance, Fitness for Life: An Individualized Approach , 6th ed (Boston: WCB / McGraw Hill, 1997).