Let's get one thing straight before we start talking about how fast you should walk: Moving at any speed is better than not moving at all. Every time you take one step, you're giving your heart, lungs and muscles a shot of health and longevity.

Now that you're moving, though, you begin to ask yourself what you can do to get more out of your walking workout. Obviously, speed is one way to pump up muscles tone, health gained and calories used.

I'd prefer walkers look at health benefits and didn't count calories used on a workout because -- as you'll see in a minute -- the total advantage gained can disappear after one sliver of cheesecake or even a healthy turkey sandwich. Take a look at my story "Weight Loss & Exercise" to check out the physiology of exercise’s benefits besides calories burned.

But if you really gotta know, here’s the scoop on speed:

When it comes to pace, remember, increases in calories used are exponential, not linear. For example, going from a 20-minute mile to a 15-minute mile, you'll use 30 percent more calories per minute. Increase your pace from a 15-minute mile to a 12-minute mile, and you'll use 50 percent more per minute. Take it from a 15-minute pace to a speed-demon 10-minute pace, and you'll double calories used per minute. (My book Walking Fast has a section on calorie use.)

Of course, as you go faster, it takes less time to cover the mile, so you have to stay out there the same amount of time, increasing the distance as you get faster.

Note: Runners don’t get these exponential increases because it takes about the same amount of energy no matter how fast you go. Walkers have to work extra hard to actually stay on the ground rather than leap into the air into a run. For anyone who’s tried to walk really fast, you know that point where your body says, "Forget this! Let’s run!"

Just to satisfy your curiosity, then, here are the approximate numbers. Remember, calories used vary greatly from person to person depending on weight, body fat, fitness level, personal metabolism, terrain, weather and all kinds of other things. The numbers calculated below apply to an 150-pound person. For every 25 pounds more or less, add or subtract about 15 percent.

Fat Burning? Forget the myths about going slower or at a lower intensity to burn more fat. Yes, your body chooses a slightly higher percentage of fat as fuel at lower intensities, but the total number of calories is also lower, as you can see above. Walking at the highest intensity you can comfortably stick with for the distance without hurting yourself will use more total calories and, in the long run, more total body fat.

If you don't have a lot of time for a walk, at least make those few minutes really hearty ones to get the best workout you can.