Overstriding is a habit many of us have had for a lifetime, because we never learned the proper walking stride. When you overstride, your front heel is hitting the ground far in front of your body. Many of us do this in an attempt to walk faster.
A good, fast walking stride is just the opposite - your front heel strikes close to your body while your back foot remains on the ground longer to give a powerful push-off. This push off in back is what will give your walking more speed and power, and will work out your muscles better to burn calories.
At first, you may need to shorten your stride and just take shorter steps. Then start concentrating on feeling your back foot and getting a good push off with it with each step. Concentrate on this for a few minutes each treadmill session until it becomes more familiar. Soon you will be walking faster and easier.
I have fallen into the overstriding habit by walking with friends who are taller and have a longer stride. I have to remind myself that the power is in the back foot, and to lengthen my stride in back and not in front.