We all know that physical fitness is a good thing. Fitness is associated with heart and lung health, good blood sugar control, and a longer, more productive and enjoyable life. But what does it actually mean, and what do you have to do to get and stay fit? Although some definitions of fitness also include muscle strength, flexibility and body composition, fitness usually refers to cardiovascular fitness, or aerobic capacity—the ability of your heart, lungs and circulatory system to supply oxygen-rich blood to muscles and organs and remove metabolic waste products during exercise. You can assess your own aerobic capacity to some extent by how winded you get climbing stairs, lifting heavy packages or running for a bus. A more scientific home test is given here. You can also go to a clinic or health spa and have it measured by exercising as hard and long as you can on a treadmill while your heart rate and breathing capacity are measured. According to studies done at the Cooper Inst…