Since you are active, I multiplied your weight X 15= 1,500 cal's. You will MAINTAIN your weight if you are taking in that many cal's, so dont worry about gaining weight. You would have to stop working out and eat more cal's to GAIN weight. Dont forget that when your menstrual cycle comes around, you may gain a few lbs (water retention). You are doing exactly what I am doing, an hour M-F of aerobics

I am 5'7 and weigh 123lbs and I do eat whatever I want on the weekends but watch what I eat on the weekdays BUT everyone is different!