I am working on my Masters Degree in Education after changing careers. For the first year, I was a substitute in my local school system (Urban – 3000+ students) working from Pre K to 12. Having this experience I found that my comfort zone was is Pre K to 6. This year I am a Reading First Paraprofessional in an elementary school and I am very happy. This opportunity came to me because I was exposed to the administration and they came to me when the position was available. Had I not been a substitute, I may not have realized this and I am sure I would have gone back into the business world and not pursued my passion for teaching.
Here is my only caution… If you work in a very large system like I have, substitute teaching can be very challenging for many of the reasons you have stated. I found substitute teaching rewarding because you will experience all types of behavior and challenges. It will cause you to reflect if teaching is the right choice for you or not, because if you can survive substitute teaching, you can survive anything. Remember, if you are in a challenging school district, the kids will more than likely be VERY unruly, this will truly test you!