Less Calories Burned Now That I Weigh Less

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Im sure that everyone started off with 30 minutes of exercise. Now that I'm already on 1200 net calories a day, the only thing to do is increase exercise time. I've upped it to 45 minutes a day but im not burning the 600+ I used to, and want to continue with that many calories without doing 2 hours or something crazy of exercise. Does anyone have tips to help me out?