How long would it take to lose 13 inches off my waist.?

i recently started exorcising 35 min pilates mainly. i started cause i just found out i am now considered obese. im 23 years old 40 pounds over weight at 5 6". i have lost two inches around my waist in about a week and a half but i know not to expect this every time i go to measure my self. i just wanna be fit for my little one so i can keep up with him and stay here on earth longer with him. i don't want to not care about what goes into my body anymore and how its going to affect my life and my sons. anyways if you can just let me know what i should be doing to lose the thirteen and how long it should take. i want to get there as fast as i possibly can without ruining my metabolism. oh one more thing i played with bulimia and anorexia a lot in my younger teen years will it affect me losing weight now.