Exercise question

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Okay here it goes, I have been watching my calories etc but I am gaining the weight I lost back. We have a fitness center here on site and am thinking that if I do 30 on the elliptical and 30 on the treadmill I should burn about 400 calories a day. Is that too much or could I add maybe swimming to the mix for 30 minutes ? I really would like to lose the weight without resorting to pills but at this point I am starting to feel desperate :-(