When you start to exercise, your skeletal muscles squeeze blood from the veins near them to your heart. Your heart muscle has to squeeze harder and faster to pump the extra blood to your body. The harder you exercise, the more blood returns to your heart and the harder your heart has to work to push it out, so your heart has to beat faster. Fitness is determined more by how hard you exercise than by how long. Competitive athletes must exercise intensely.
Runners have to run very fast in practice and weightlifters have to lift very heavy weights.
You will become more fit by running a few miles fast once or twice a week than you will be by running a lot of miles very slowly. However, every time that you exercise intensely, your muscles are damaged and it takes at least 48 hours for them to heal. That's why competitive athletes exercise intensely 1 to 3 times a week and exercise at a more casual pace or take days off the rest of the time. If you want to become more fit, you need to do the same. I'm Dr. Gabe Mirkin on Fitness.