I teach the Precalculus at the University of Washington. I teach it a lot, along with other courses. I have taught it every fall, winter and spring quarter for a long time: Winter 2005, almost five years ago, was the last such quarter I did not teach it. I feel burned out with it, though I just made a lot of changes to the course content over the summer, and so this quarter is fresher than it would otherwise be. Still, I can feel a definite lack of energy and pep in my thinking about the course.
On the other hand, my precalculus lecture this quarter is going quite well. The students seem more energetic than usual, and the atmosphere is better than I can remember having in some time. I usually find that it just takes a few students to add a bit of energy to the audience to really lift the whole class. I definitely have a few of this sort of student this quarter. One of the changes I made over the summer was to remove three chapters (of 23) from the textbook, so we have less material to get through, and the reduced pace may be contributing to the positive feeling.
Still, it's only fall quarter. I just had months off for summer: should I feel so burned out already? Can I keep doing this for the next 20 years until retirement? Do I want to? What else might I do?