Back in graduate school when I was a teaching assistant, I remember asking a professor how long it took him to prepare for class. "About 10 minutes," he said. I didn't understand. I was in the process of learning how to put classes together myself, and each session for an Intro course would take me several hours to prepare. Sometimes it would take all day. How in the world could someone stand up there for an hour in front of students with only 10 minutes prep? It helped that he'd been teaching the course forever, so he would take his already prepared notes and review them before class. Observing him as his assistant, I could see him go off the cuff and comfortably field questions from students. He'd taught this material 100 times before.
I get bored easily. I can't teach the same material over and over. I retool and update my courses constantly. So I'm not in the practice of reusing one set of notes each time I teach a course I've taught before. But I do have lots of notes that I can work from when doing class prep. I still start from scratch, often, but I also can count on a library of examples and notes that have worked well before.
The main thing for me these days, when possible, is to finish my class prep as early in the day as I can. When teaching on a M/W/F schedule, as I'm doing this semester, I try to have my class prepped the prior day as early as possible. That way I can spend portions of the rest of the day thinking about how I'm going to teach the material and thinking about new questions to ask. It's less about the feeling of being overprepared with more notes than I need. It's more about thinking how I can present the material in a way that will engage students. And it's always about composing questions in my head that I think students will be interested in answering.
I'm thankful to my students for always inspiring me with questions of their own, and for supplying me with new examples to consider. Even while teaching these days masked up with a sweaty face and struggling to hear students in the back of the room, it's a joy to establish connection with students and to see them thinking through the material.