I will tell you as a previous teacher my hope that I wish I didn't have was that I was going to teach my students math and that they would understand because I was going to teach them the "how". This is an unrealistic goal (particularly for a beginning teacher). This pessimistic view will probably be met with down votes, but you need to hear it as a beginning teacher. You should lower your expectations.
Even if you are a great teacher, it's unlikely that you will be successful for your first few years. You need to lower your expectations. You need to both lower your expectations for what you think you can teach and, especially, lower your expectations of what your students already know. You are likely going to have to reteach many topics that you think are prerequisites to your course (whether it be geometry, pre-cal, algebra, etc.). You cannot teach assuming your students already understand/know the requisite skills.
Assuming this is for high school/junior high education, the thing I wish I had been more knowledgeable about was classroom management. This is far more important that your knowledge of mathematical topics. If you assume that you are going to keep your students' attention by explaining things in the most perfect way then you are seriously mistaken. High school and junior high students are not in your classroom to learn about math, they are in the classroom because they are forced to be there and I guarantee that a majority of your students don't want to be there.