Sustained physical activity requires that your heart pump blood to the lungs and to the exercising muscles. The muscles extract oxygen, sugar, and other nutrients for use in chemical reactions that enable the muscle to keep moving (contracting). To say that someone is physically fit simply means that the heart easily pumps a large volume of blood and the muscles extract and use nutrients very efficiently. The heart, after all, is just a hollow muscle that pumps blood. If you stimulate your heart muscle through exercise, it will become more powerful and able to pump more blood. Regular sessions of physical activity increase the metabolic efficiency and power of your other muscles, too. There are various degrees of fitness, with professional and Olympic athletes at the extreme upper end.
I’ve had otherwise healthy overweight patients so “out of shape” that walking 20 yards to the mailbox was a real chore. They were tired and panting when they got to the mailbox and had to rest a bit before returning to the house. These folks are habitually sedentary and dramatically overweight. But you need not feel too sorry for them. After starting and maintaining an exercise program, these unfit people achieve the greatest degree of improvement in fitness level. They make more progress, and faster, than those who begin with a greater level of fitness.
The way to achieve aerobic fitness is to regularly challenge your large muscles to perform sustained physical activity. “Regularly” means at least four days a week, if not daily. Left alone, your muscles don’t want to do much other than just get you through your day comfortably, without effort or aching or cramps. You must challenge them to do more, work a bit harder, tolerate a little aching. You’ll know you’re challenging them during exercise when you perceive that mild to moderate effort is required to keep the activity going. You should be mildly short of breath, perhaps even perspiring lightly, yet still able to converse. “Sustained” physical activity means at least 30 minutes in a day. Most people find it a better use of their time to exercise for 30 minutes continuously rather than break it up into five or 10 minutes here and there.
Discontinuous activity (e.g., 10 minutes thrice daily) probably is just as good. If you think about it, there are many easy ways to increase your discontinuous physical activity. Consider taking the stairs instead of the elevator, parking far from the supermarket or workplace doors, walking the golf course instead of riding a cart.
(The exercise model above is “old school,” which isn’t necessarily good or bad. Some newer scientific studies suggest that you can achieve comparable levels of fitness with much less time exercising, if you do it intensely. An example is high-intensity interval training (HIIT). That’s worth a blog post or two by itself. I also leave strength training—also an important aspect of fitness—for another day.)
If you’re starting out in poor shape, you won’t be able to do 30 minutes of any exercise without adverse effects. Don’t even try. The worst thing you could do at this point is injure yourself or have such a horrible experience that you give up entirely. Thirty minutes of daily activity is your goal to achieve over the next four to 12 months. Moderate to high levels of fitness will take you six to 24 months. The most important thing when getting started is to exercise at least a little, five to 10 minutes, on most days of the week. And don’t overdo it in terms of intensity. Start low, go slow. After three months, exercise will be a habit. Prolongation of your exercise sessions will be easy as your amazing body responds gradually to the workload through the process called physical conditioning.
If walking 30 minutes daily is too hard for you at first, try walking just an extra 10 or 20 minutes daily. If you can do that but it’s a bit of a strain, gradually (every two weeks) increase your walking time by five minutes daily until you are up to 30 minutes. Average walking pace is 2 mph (3.2 km/h). Once you can comfortably handle 30 minutes daily, the next step is to increase your walking pace to 3 or 4 mph (4.8–6.4 km/h) for the entire 30 minutes. Four mph (6.4 km/h) is definitely a brisk walk. It’s difficult for many people to sustain over 30 minutes until they work up to it gradually. This is often done by walking at two paces, normal and brisk, during an exercise session. You might walk five minutes at normal pace, then five minutes briskly, alternating every five minutes until the session is over. Every two to four weeks, you can increase the minutes of brisk pace and taper off the normal pace. You’re able to do this easily because your level of fitness is increasing.
I’m asking you to walk briskly (3–4 mph or 4.8–6.4 km/h) for 30 minutes most days of the week. This brisk pace burns roughly 200 calories per session, in case you’re wondering. If you eat a 400-calorie muffin, it provides enough energy for a one-hour brisk walk. If you don’t burn the muffin calories as exercise or basal metabolism, they’ll turn into body fat. (But you’re not eating muffins anymore, are you?!)
If you prefer physical activity other than walking, the general rule is to start slowly and gradually increase your effort (intensity) until you’re up to about 30 minutes of moderate-intensity exercise most days of the week. Start low, go slow.