Maeve, even with preparation, it takes several years to become a truly effective teacher. The first year is spent making "new teacher mistakes." We all do this. We learn from these mistakes. The next year, we take our hard-won learning, and put it to work. The third year, most of us can teach, manage a classroom, get the paperwork done, be a social worker, etc.
To be a teacher, in addition to knowing your subject material, you need to make a lot of decisions on-the-fly. You must multi-task like crazy. Knowledge of student's lives, ways of learning, preferences, best-practices of teaching, and developmental theory must be embedded in your psyche. It takes time for this to happen.