You'll be surprised how quickly your body adapts! Whilst I don't do the gym, I've got equipment at home, and seriously struggled with 2 minutes on a cross trainer when I started, now I can do 30 mins straight and some serious exercise (weights, crunches, squats etc) with no problem, I went from nothing to about 45 minutes every day with not a lot of hassle, don't push yourself too hard, but remember when you start to struggle that's the work that counts the most and will cause your body to change.
Good luck!
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