Aerobic exercise (cardio) is activity that your body does using oxygen as fuel. So as you condition yourself through cardio exercise, all the components that go into oxygen delivery and usage become more efficient.
Follow the path- your breathing becomes more regular and you can take in more air with each breath. Your lungs become more efficient at extracting the oxygen and injecting it into the blood stream.
Your heart becomes stronger in increasing its stroke volume and cardiac output. This means the fit heart pumps more blood per beat than the unfit heart, and so doesn’t have to work as hard.
The mitochondria in your muscles become more efficient at extracting and using the oxygen brought to it by the vascular system. During the aerobic process, glucose and fatty acids found in the blood stream are also used up as fuel. This means the carbs and fats that you have eaten, but have not yet been stored as fatty tissue, are used up.
The above is a very elementary explanation of what happens. It’s a process. An unfit person would start at maybe 15 minutes of cardio exercise, but eventually work up to 45 minutes. Each type of cardio exercise has its advantages. The low intensity, longer duration (walking) (40-60% of MHR) leads to increased blood flow and general health benefits. It begins the fat loss process.
Medium intensity and duration (around 70%) improves aerobic capacity and increases cardio and respiratory endurance.
High intensity with a shorter duration (80-85%) gives the biggest metabolism boost and the largest increase in cardio and respiratory endurance. If you can only do 10 seconds of sprints or running at first, you will eventually work up to minutes instead of seconds.
Higher than 85% and you get to the anaerobic threshold. This is when your body produces lactic acid and it learns to deal with this, thus increasing your endurance and efficiency. For the average person looking for general fitness and weight loss, this level is difficult to achieve and maintain. I did reach it and worked at it for about 10 months, but for the last year, I haven’t gone back to it.
The whole thing is a process. To get from point A to point C, you start where you are now. Work at different levels to take advantage of the different benefits. For medium intensity work, increase your speed and distance or time each week. For high intensity, increase your speed or number of intervals each week. Each person is different. When I do increase my high intensity work, I usually stay at that level for a week or two, because my body needs time to adapt. A younger person might not have to take that time.
This is a long answer, but I hope it makes it a little clearer for you. The aerobic system is the same whether you exercise or not. Exercise just makes it function more efficiently and stronger.
Do or don't do; there is no try (Yoda)