Intervals are workouts that include alternating segments of high and moderate intensity exercise. Intervals are incredibly effective, and anyone can do them because the intense segments only have to be harder than your typical workout, and not as hard as, say, what a professional athlete might do.
For example, if you are someone who typically walks at a snail's pace, then upping your intensity could mean walking at a turtle pace. The important thing is that you're challenging yourself to work harder than you typically work. These intervals give you a way to increase your overall walking pace, while, at the same time, increasing your caloric burn and cardiovascular strengthening during one workout.
Here's another great thing about intervals — they don't have to last long to be effective. Even 30 seconds at a faster-than-normal pace will do your heart and your calorie burning goals a whole lot of good. Of course, the longer and harder your exercise, the more effective the workout is, but, once again, if you're not in terrific shape, even a 30-second interval can improve your heart health, up your fitness level, and increase your caloric burn exponentially.
To find out how fast you're walking, count your steps (both feet) for 20 seconds (use the second hand on a watch, or a stopwatch). If you count 40, you're walking about 3 mph (which is good for your health, but not your fitness). If you're up to 45, you're walking 3.5 to 4 mph, which will lead to weight loss. Get up to 50, and you're walking to improve your heart.
There's one more great thing about interval workouts: You can increase your intensity in a variety of ways. For example, to make segments of your walk more intense, you can jog, climb a hill, add a weighted vest, or walk faster.