I feel you. I'm a former preschool teacher but I will be working with elementary age children in the fall. Those little ones take a lot out of you for sure!
I know it's hot, but can you run during lunch instead of walking? I did that for a while and it really helped keep me on track and energized me for the afternoon
Sometimes when you're dead tired you just have to do things anyway. I dragged myself off to the gym sooooo many times when my mind kept telling me I was so tired and should just stay home but I suddenly found the energy to work out when I was there!
Good luck, I hope you can stay on track and I'm sure it will not be as difficult as you think it is!