yeah maybe...i am a half miler and it is not nearly as physically demanding as cross country was for me. not to say that the distance events are harder but it depends on the person; their physical composition and training will make the difference.
2nd of all i think you can eliminate the 800 because it does not even win for being the most difficult middle distance event...that would be the 1k. it is run very similar to an 800 and i think it requires the same range of abilities, but it is longer and quite the brutal race.
of the shorter distance events (3k-10k) i think that the 3k steeple would be the most difficult. i would say that i am not the best to argue here as i have never raced over an 8k and only have 1 steeple under my belt, and it was the worst day of my life.
next up you got the long races and i am soo ridiculously impressed by those guys who run the marathon. this is probably a solid combination of the fact that i am weaker in distance races and the it is just ridiculous to be able to push ones body that far.
of the sprints i would say the 400 hurdles is the most demanding. while i take no credit away from the athleticism or greatness of the accomplishments by sprinters, i believe there to be little argument in favor of any of these events being the most demanding so we will discount them.
so in my eyes we are down to the 1000 (if we are considering all events, as i ran this since i was naia and many of you may not have, otherwise we will use the 800), 3k steeple, and marathon, with my vote being the marathon.