There are many benefits of exercise beyond improving cardiovascular health. In general, any exercise is better than none, more is better than less, and different types of exercise such as aerobic and resistance yield different benefits.
There are several ways of quantifying training intensity. One approach is to calculate the percentage of the maximum predicted heart rate, derived as 220 minus age in years. For example, the maximum heart rate for a 50 year old person is 170 (220-50=170). Training based on target heart rate can be adjusted based on the heart rate at rest. For example, a 50 year old with a baseline heart rate of 70 will have a heart rate reserve of 100 (170-70=100). Training at 50% of heart rate reserve in that individual would be at a heart rate of 120 (70+0.5x100). Another approach to measure training intensity is the Borg Perceived Exertion Scale, which provides a relative perceived exertion index 6-20. (Table below). With this system, training with the perceived score of 12 to 16 correlated consistently with a 40% to 80% heart rate reserve.
According to the AHA guidelines, the ideal training goals for endurance training are to exercise at least five days a week for 30-60 minutes, depending on the intensity. Obviously this goal is not attainable for many patients, and a more modest goal is more realistic. The optimal intensity is 55%-90% of maximum predicted heart rate, or 40%-80% of heart rate reserve, or a perceived score of 12-16. This can be accomplished by walking, jogging, treadmill, cycling, etc.
For patients with coronary artery disease, valvular heart disease, congenital heart disease, cardiomyopathy, arrhythmias, a more in-depth discussion with your cardiologist is needed to make the final recommendation.