It could be your shoes (it helps to invest in a good pair of walking shoes). Do what you can in terms of walking, but the key is to make sure that you are under your calories for the day. It is much easier to lose weight by controling the input (the food you eat) than the output (the exercise you do). Exercise is important, but it will get easier over time as you drop weight.
As you lose more weight, exercise (walking) will be easier so you can walk more. As you walk more, you will lose more weight.
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