Because I am more fit then most people and its not bragging when 1/4 people in the US are obese, its common sense. How do I know I could finish ahead of most people? Well I work at a private gym part time training personal clients at night. Maybe becasue I invite the guys who do these from my Office to come do a free cross fit session we do at the gym on saturdays? Maybe I go to these bro fests and hand out flyers to promote the free crossfit session? Maybe Ive turned a couple of these guys into my clients?
Having participated in the one last year at Mount Snow, and am registered for the one in May somewhere near Boston, I can assure you that these events are about much more than just physical fitness.
I went into the event with the same attitude as you, "Oh, I'm in great shape and have been training for the majority of my life.. this is going to be cake." Well.. that's just not true. As previously mentioned, you don't run consistently that much in an event like this.. and sure crossfit will definitely help with some of the obstacles... but regardless of your strength and conditioning, you will be tired.
Events like the toughmudder are designed more so to test your character and mental toughness. Every bit of water you touch is ice cold and puts you at risk for hypothermia.. Not only that, but imagine crawling through ice cold muddy water with live wires (up to 10k volts) hanging above you which are pretty much unavoidable.
It's a hell of an experience and I recommend it to all.