The two main times i lost a lot of weight as an adult, i had to exercise like 10 hrs a week, or 90 minutes a day - like walking 6 miles a day, or swimming, weight lifting, and stationary bike, a few times each every week. 30 minutes a day is like the minimum to maintain health. Like you just break even. You dont start burning fat until at least 45 minutes into a walk. The last 45 minites is what accomplishes what you want.