Gabe Mirkin, M.D.
Most exercisers don't have the foggiest idea what fitness means or what they have to do to become fit. Fitness refers to your heart muscle. The stronger your heart, the more fit you are.
The only stimulus that makes any muscle stronger is to exercise that muscle against increasing resistance. You do not become stronger by lifting the same weight day after day. To make your skeletal muscles stronger, you have to lift heavier weights or press against greater resistance on weight machines. The only way that you can strengthen your heart muscle is to exercise against greater resistance also. No drugs strengthen your heart muscle.
When you start to run, your leg muscles squeeze blood from the veins near them toward your heart. Then, when your leg muscles relax, the veins near them fill with blood. This alternate contraction and relaxation of your leg muscles acts as a second heart pushing huge amounts of blood towards your heart. To pump the extra blood from your legs to your heart, and then to your body,/your heart muscle has to squeeze harder and faster. The harder you exercise, the more blood is pumped by your legs to your heart, and in turn, the harder your heart has to work to push it out towards your body, so your heart has to beat faster and with more force to do more work.
Fitness is determined more by how hard you exercise than by how long you exercise, because the harder you exercise, the more blood returns to your heart, and this increased amount of blood fills the inside of your heart and stretches it, so your heart has to pump against greater resistance and therefore becomes stronger. Competitive athletes know that the only way that they can become stronger and faster is to exercise their muscles against greater resistance. Runners have to run very fast in practice and weight lifters have to lift very heavy weights.
You will become more fit by running a few miles fast once or twice a week than 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 one to three 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. On one day, you have to injure your muscles by running, biking or rowing faster or lifting heavier. The next day your muscles feel sore because they are injured, so you have to exercise easily or take the day off. And you continue to exercise easy until your muscles do not feel sore. Then you exercise intensely again.