20 pounds would put you at 2 a week, which IS doable, but not guaranteed. You may have weeks where you lose 6 and some where you lose 1 or 0, even when you are 100% on plan. It really will be determined by how disciplined you are, how your body reacts and so on. Best of luck! Stick to it, either way, you will be better off.
"I do feel that spiritual progress does demand at some stage that we should cease to kill our fellow creatures for the satisfaction of our bodily wants." -Gandhi