Teach, I think we were banded on the same day, and I also work in a school, though not as a teacher, a sign language interpreter. Kids are exhausting.
I started working out a few days ago - walking on a treadmill for an hour. I think the only difference between us is what we're eating. If I were only consuming liquids, I'm sure I wouldn't have the energy to excercise. I'm lactose intollerant, so many of the protein drinks aren't an option for me. Instead, I've been eating lots of omelets. Hopefully, your doctor will let you eat something more substantial so you'll feel more energetic.