Janelle,
I agree this can happen without careful planning. Like you, I approach a class with what is most important that I want students to get, planning that material might take half the class time. But if class does not go as planned, interuptions occur, more student questions, or I need to explain more or review, I know I will have planned for enough time. With experience, these events become easier to plan and map out the day's activities.
Barry Westling
