Teaching to the plan

I've had the opportunity to teach several classes now.  Frequently, my cooperating teacher will share what she had in mind for the class period and ask me if I'd like to teach.  I enjoy working with students and sharing analogies that I think will help them, so I usually jump at the chance to teach.  The problem is, what she has in mind does not usually get completely transferred to my brain before I start to teach the class.  With algebra, the textbook is more traditional and I'm comfortable with all of the ideas and the overall learning target for each day.  However, with CMP2, I feel that the text does not adequately cover the main ideas and we need to supplement with additional information.  Twice, I have taught the CMP2 class and discovered that I skipped an activity that my cooperating teacher had in mind, but was not clear to me.  I'm learning first-hand that it's important to thoroughly plan beforehand!