The colleagues I really envy are the ones who get to sit during their lectures. I teach three classes in a row on Tuesdays and Thursdays, and by the end of the day my feet hurt so much I can barely move. All my classes are a mix of lectures (during which I stand and walk in front of the board) and group activities (during which I trot around the room, trying to maneuver between chairs, tables, huge backpacks students love carrying around, coffee mugs, and water-bottles.) Sometimes during group activities, questions from students come so fast that I have to jog around the classroom, jumping over stuff they throw on the floor. At the end of my last lecture, I always wonder how on Earth I will be able to walk out of a classroom. Today, I had to illustrate the Spanish word "cojo" (which means "lame"), and that was really easy.
Right now I'm sitting on the sofa, contemplating with horror the eventual need to get up.