Classes are generally rated as beginner, intermediate, and advanced. Choose the level that fits your condition. It's no fun taking an advanced class if you're a beginner. It will be hard and frustrating and you won't enjoy the experience. Watch the class or speak with the instructor to help you decide what's right for you. Sometimes it comes down to the class time that fits your schedule, but just be sure to not get in too far over your head.
If you want to burn more calories while walking, the metabolic equivalents (MET) research that produced these calorie numbers show some ways to do it. You will burn more calories by walking more miles. Going faster at a walking pace has little effect on the calories burned per mile, but it can make a difference because you'll cover more distance in the same amount of time. If you only have 15 minutes or 30 minutes for walking, then going faster is a good strategy.
Not so much. In fact, running more than doubles the amount of energy you expend versus walking, according to research published in the journal Medicine and Science in Sports and Exercise. A 140-pound person burns 13.2 calories per minute running, according to the American Council of Exercise. That same person would burn 7.6 calories per minute walking. I’ll do the math for you: For a 30-minute run, that works out to 396 calories burned running compared to 228 calories burned while walking for 30 minutes.
Breathing increases during aerobic exercise to bring oxygen into your body. Once inside your body the oxygen is (1) processed by the lungs, (2) transferred to the bloodstream where it is carried by red blood cells to the heart, and then (3) pumped by the heart to the exercising muscles via the circulatory system, where it is used by the muscle to produce energy.
Stay hydrated. Drinking enough water is an important part of a walking workout. Remember that you lose water through sweat even in cooler weather and that you don't start to feel thirsty until you’re already starting to become dehydrated. Drink about two cups of water before you start and another cup about every 15 minutes. Don't wait until you get thirsty.