Fitness Instructor on Speed

I signed up for a kettlebell class, and I got a circuit.  The instructor was a real task master, I was in her class once after my surgery, and it was not fun; as soon as I saw her, I knew I would be knackered the next day.  I managed to stay the whole class even though I was really thinking of going half way.  How could she still have energy jumping after the whole session provided she already led another session before my session?!