15-20lbs in 6 months is certainly do-able. The biggest thing I can tell you is to make sure when you are logging your activities make sure that you log sleep. How ever many hours of sleep you got....log it! You may see a difference in your burn vs intake that way. I know I do. The other thing to keep in mind is that in general burning 500 calories more than you eat per day will yield a 1 lb per week fat loss. Unless the laws of nature decide to play with your scale.