It is great that you walk your dog but how fast do you walk? Also, just like for people, we gain weight gradually and we should loose it gradually. Always measure the amount of food you provide. Make sure you feed her food that she likes. Then, reduce the amount of food that you give her. Do this in measured increments. Do you feed your dog about 2 cups per day. For the first week, reduce the measured food by 1/2 cup per day. You might not see a change over the first week. For week 2, reduce the food by another 1/2 cup. You should see a change now. She should be hungry. Remember that in order for her body to use up stored fat, she needs to be hungry. You may want to feed more often by dividing the daily measured amount into two or three feedings daily but maintain the total measured amount.
Try this method with the guidance of your vet.