The first time through new material is tough, the second time is easier, but it usually takes me three times before I feel comfortable with the lectures.
It certainly is easier further on down the road. Fifteen years later most of my lectures were quite familiar and comfortable but still subject to changes with the changing world of computing around us.
I suppose statistics and math would be subjects with only minor changes over the years - how about astronomy? Do things happen often enough that you would have to revise theories and stuff?
PS. Most of this is now behind me since I retired recently ;)