No way the 800 is the most difficult. You're really only hurting the last 45 seconds, anyone can suck it up for that short of a duration.
5k's are horrible, and 10k's are even worse. I can't even imagine what a marathon is like, and I will never try. Think about it this way - the last 1/3 of any race is going to hurt. I'll take the shortest pain possible. You crazy long distance runners who can suck it up for 10, 15, 20 minutes or more of intense pain - my hat is off to you. I'll stick with my 800's, they feel easy in comparison.