Your weight is primarily determined by your balance of calories, meaning how what you burn stacks up against what you consume every day. In order to lose weight you are going to need to increase your daily activity so that you are burning more calories, or to decrease what you eat so you are consuming fewer calories.
A single pound of fat amounts to 3,500 calories. If you want to lose one pound of fat every week then you need to expend 3,500 more calories per week than you consume, which you can do by decreasing what you increase, increasing what you do or both. A sensible goal is to lose between one and two pounds per week, so you should combine an increase in activity with a decrease in what you consume throughout the week in order to lose 3,500 calories every 7 days.
It is important that you understand how you burn calories. Your weight multiplied by the distance amounts to the energy that you are using while you walk. The time does not actually matter as much as the distance does. If you speed up to the point where you are walking a mile in less than 13 minutes, then you are going to be burning more calories for each mile that you walk. For more walkers who are just beginning, it is best for you to increase the distance that you can travel before you begin to worry yourself over your actual speed. A typical rule of thumb that you can follow is that you will burn 100 calories with each mile that you walk if you weigh 180 lbs.
You are going to burn more calories for each mile at lower speeds because momentum is not carrying you along the way it does when you are going more quickly. At high walking speeds, however, you use more muscle which means more calories so there is a bit of a balance depending on the speed you use.