Sounds about right! I think the fear/flight instinct should have kicked in some morphines to totally ignore the pain while you were doing it, but yes ... you would feel knackered afterwards. Great! Push yourself beyond every so often. I'm going to try to stimulate the fear instinct.
While out in my huaraches the other evening (I mention that because I think the made a difference to how I sound to animals), I set off on my warm up fast run and fixed a position to break into sprint. I did this and to my surprise I was surrounded by rabbits who were so startled to see a human in the middle of them that they froze, scattered and ran alongside me. I actually scooped one up like an errant rugby ball! I was so surprised myself, I fell over, let the little fellow go, rolled, back on my feet and was running backwards before I knew what was going on! It was utterly bizarre! Stepping outside of normal bounds is quite surreal.
I had to stop as well - I felt faint and quite unreal. I guess it was the adrenaline, morphines and so on coarsing around the body. No, your heart won't explode, but I bet that feeling will get addictive. I'll definitely try the fear factor.