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. To make your skeletal muscles stronger, you have to lift heavier weights or press against greater resistance in any weight-bearing exercise. The only way that you can strengthen your heart muscle is to exercise against greater resistance also.
When you use your legs, 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. Exercising at a casual pace does not do much to strengthen either your heart or your skeletal muscles. When you work harder, 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 the heart muscle becomes stronger.
The latest research shows that exercising for 30 minutes three times a week will not necessarily make you fit, nor does exercising for 60 minutes seven days a week. Exercising at a casual pace does not strengthen muscles. This means that going out and jogging slowly so that your leg muscles are always comfortable and do not burn will not make you fit.
When you exercise intensely, your muscles stretch and tear. It's the tearing that causes the burning during exercise, and leads to the soreness that you feel for the next day or two. When your muscles heal from these tears, they are stronger than they were before. So it's the burning during exercise that causes the tearing that causes the next-day soreness. Then you take days off or go slowly so you can recover; your muscles heal, which gets rid of the soreness, and with healing, the muscle is stronger than it was before you did the exercise.
However, there are some serious problems with training for real fitness. If the force on your muscles during exercise is greater than the strength of your muscles, they will tear too much and you will be injured. If your muscles are still sore from a previous workout and you try to exercise intensely, you can cause a serious injury. You must learn to tell whether the burning is the good burning that causes muscle growth or the bad burning in which you put too much force on your muscles and tear them so you can't exercise at all.
The program I recommend for fitness applies only to healthy people. It could cause heart attacks in people with damaged hearts. Before trying this, check with your doctor. The rules for fitness are that you should spend several months exercising at a casual pace and not going for the burn. After a few months, you should be able to exercise 30 minutes every day and not feel sore. Then you are ready to start training.
If you are a runner or a biker, go out and run or ride very fast until your legs burn, slow down until the burning goes away. Then when your muscles feel fresh again, pick up the pace. When your legs start to stiffen, stop the workout. On the next day, either do nothing or go slowly and do not try to do another intense workout until your muscles feel fresh. Then when your muscles feel fresh, you take another hard workout. Remember, trying to exercise intensely on sore muscles will only injure you.
