Nutrition. Some say it's the fourth discipline. I believe I have written about this before. Maybe more than once! For me it's the first discipline. In three Ironman races, I managed to get it right only once, my first time. I followed a similar program the very next year but the results were quite different. Let's just say it didn't work. By the time I left the transition tent after the bike, my stomach was well on its way to shutting down. After dry heaves at mile 10, it was "the will to finish and flat coke" that got me through. I walked most of the last 13 miles! Not fun! Some say that if one walks in Ironman, then there is something not completely right about that. I disagree. Triathlon, and Ironman specifically, is about finishing. As long as you do it legally.
But getting back to what makes this race so hard, Nutrition is second on my list. If you don't hit it right on the bike, it makes the run that much more difficult. And you are in for a long day. Trust me, I've experienced it twice now.