There does seem to be a bit of an overreaction on her part.
But on the other hand you know you were doing something potentially wrong as well if you had to hide something and enlisted your family and friends to help keep it going.
Just remember, the issue at hand here is not going to be the smoking, it's going to be the fact that you hid it, the length that you hid it, and that other folks were involved in hiding it. All of that is going to make her feel like an idiot. If you are going to repair this you have to go at it from that angle, say you understand how made her feel, thats why you got it off your chest, because it made you feel awful having to hide it as well.
But on the other hand you know you were doing something potentially wrong as well if you had to hide something and enlisted your family and friends to help keep it going.
Just remember, the issue at hand here is not going to be the smoking, it's going to be the fact that you hid it, the length that you hid it, and that other folks were involved in hiding it. All of that is going to make her feel like an idiot. If you are going to repair this you have to go at it from that angle, say you understand how made her feel, thats why you got it off your chest, because it made you feel awful having to hide it as well.