Only a few decades ago, running was considered the sport of oddballs and kooks; no one recognized its health benefits. Although today it is widely accepted that fit people are healthier and live longer, it took some rigorous research to prove the point.

Researchers J.N. Morris and Ralph Paffenbarger proved in the 60s that the fitter the specimen, the longer it lived. But that left an important question: Why? To appreciate the answer, you need to understand a little bit about how the human body operates.

The aerobic and anaerobic systems

The word “aerobic” means “in the presence of oxygen.” You are operating aerobically when you walk, sit, sleep, eat, watch television or read a book, and, ideally, when you exercise (“ideally” because when your body operates aerobically, it can produce energy to keep you going for a long time). In the simplest terms, you produce energy aerobically when the air you breathe works together with the food you eat to make your muscles operate. It’s similar to the way gas and air work together to make a car’s engine go.

Sometimes your body is called upon to do strenuous work very quickly – for example, to help you flee when you suddenly find yourself between a bear and her cubs. To have any chance of escaping, you will have to come up with a lot of energy almost instantly. That’s when your anaerobic system is likely to kick in. As the name suggests, anaerobic means “in the absence of oxygen.” Unlike the aerobic system, which requires oxygen to produce energy, the anaerobic system uses the fuel stored in the muscles.

Day to day, your body gets its energy from a mix of aerobic and anaerobic sources. The more intense the activity and the more you body’s demand for oxygen exceeds your ability to supply it, the more you work anerobically. That’s why when you’re engaged in intense activity, your breathing accelerates: your body is trying to get more of the precious oxygen and remain aerobic. Everybody, even a highly trained athlete, works anaerobically in certain situations. A wide receiver going for a pass in a football game, for example, will produce energy anaerobically when he is sprinting down the sideline in pursuit of the ball, As you become fitter, however, you will push up your “anaerobic threshold,” the point at which your body switches over to anaerobic-based energy sources.

The reason you want to push up your anaerobic threshold is that if you are getting your energy from mainly anaerobic sources, you can’t keep up any activity for very long. Depending on how fit you are, your anaerobic energy supply will last from 5 to 60 seconds. Obviously that’s not enough to allow you to run around the block, let alone to run 10 k.

Another reason to prolong the amount of time you work aerobically is that the chemical reactions taking place in your body during anaerobic exercise produce accumulations of lactic acid in the working muscles. Researchers think this byproduct is the cause of soreness in muscles following bouts of strenuous exercise. Again, depending on how fit you are, your body will take a day or more to break down and eliminate the lactic acid.

Feeling a little stiff and sore after a workout isn’t entirely bad; it’s part of the process that will make you fitter. Still, the 15-week walk/run program that I will show you over the next few months will slowly increase your tolerance for exercise while keeping your body working aerobically as much as possible. As you gradually train yourself, you will find you are able to function more efficiently (that is, aerobically) at higher workloads.