Part of me feels that the bitterness will not be as noticeable as you might hope. You are using a lot of "sweet" ingredients in your brew. Honey malt, Car 120, molasses, brown sugar. Even though the yeast will ferment the majority of the brown sugar and the molasses, it will still leave behind some sweet elements of those ingredients (not including the sweetness added by the car 120 and honey malt). The yeast will not be able to convert all those dextrin sugars.
Never-the-less, I am assuming that you were hoping for a beer that is more on the sweet side because you picked these ingredients. If you want to secondary ferment for hop aroma then go ahead, but don't expect the bitterness to get much higher than what it is right now.
IBU is just a guideline. Bitterness is relative to the amount of sweetness in the beer. I agree with the other posters that you should use a calculator next time just to be more accurate. Bottom line, this batch of beer should still turn out okay, it will just be on the sweet side