Some people may attack me for this post. However, I actually think you can over-prepare for lectures. I notice that if I spend too much time preparing I start to go over the material too fast and also start to focus too much on little subtleties that I found interesting.
In contrast, if I have to think through how to do a problem on the board in real time it slows me down to pace that is more appropriate for students who are encountering the problem for the first time.
I am also told students like to see lecturers sweat it out!