I have been walking 3-5 miles on the treadmill for about four weeks now. Water balance affects a person's weight quickly. With that amount of exercise, you may be getting large water fluctuations due to perspiration. Be sure to stay well hydrated and watch your calories in your diet. You may find more success by observing measurement changes rather than weight loss.
Many people find their waist shrinking, hips shrinking, yet no weight loss is apparent.
This is a positive effect of exercise. If body measurements are also not changing, you will need to pay closer attention to your diet. I am seriously considering a vegan diet. A few vitamins and minerals will need to be supplemented. Also, protein may be low, so be sure to find good vegetarian sources of the protein.