I'm 5'9/10 and i weigh about 160 and im in pretty good shape but I have stomach fat. I cant get rid of it. I swim everyday, a mile everday, and i go really hard. I've lost a lot of weight on my waist but i still have stomach fat. A freind of mine told me to start doing 500 sit ups a day and in about 3 weeks i'll have nice abs but i'm not sure? What am i doing wrong?