There is a comment in a thread of nervous first-timers that I keep coming back to, because it says in two sentences what takes most coaches a paragraph.

What destroys most Ironman days is one of two things: fuelling or pacing. It is almost never a lack of training.

That matches everything I have seen. People arrive fit enough. They leave the course anyway.

Reason One: The Stomach

The most common cause, and the one people are least prepared for.

Here is the mechanism. Digestion requires blood flow to the gut. Exercise diverts blood to working muscle. The harder you work, and the hotter you get, the less capacity your gut has to process what you put in it. Meanwhile the day is eight to fifteen hours long, so you cannot simply not eat.

So the day becomes a balancing act between two failure modes, and both are common.

The best race report I read this year was a detailed account of the second one. The athlete took an entire hour's worth of carbohydrate and sodium before even mounting the bike, on advice about fuelling around the swim. He then spent the next four hours on the edge of vomiting, fixed it by stopping intake completely for an hour and letting his system clear, and then restarted at half the rate. He finished. Most people in that state stop.

Reason Two: The First Two Hours of the Bike

The other half of the answer, and it is a decision, not an event.

You come out of the water with adrenaline high, having done the part you were frightened of. The bike course is fresh, your legs feel superb, and riders are streaming past you. Everything in the situation is telling you to go.

Riding twenty watts too hard for two hours does not feel like a mistake at the time. It feels like a good day. The bill arrives at kilometre thirty of the marathon, six hours later, and by then it cannot be paid.

The two mechanisms compound, which is why they appear together so often. Riding too hard reduces gut blood flow, so you absorb less, so you under-fuel while over-spending. That combination is the shape of most bad Ironman days.

The Two That Are Not Talked About

Two more deserve a mention because they end days that had nothing wrong with them.

Cold. A wet swim exit followed by five hours moving at thirty kilometres an hour through cold air is a serious thermal load. I have seen an experienced runner with twenty years of racing DNF a sprint triathlon from hypothermia, because he assumed a twenty-five minute bike leg was too short to matter in the rain. A packable gilet weighs almost nothing.

Mechanicals. Not the puncture. The inability to fix the puncture. Carry a tube, two levers, a CO2 canister and a spare, and change a tyre at home until you can do it in five minutes.

How to Actually Solve It

Both main causes are training problems with training solutions.

For fuelling: establish your rate in training, not on race day. Start at forty to sixty grams of carbohydrate per hour and build gradually towards sixty to ninety over months. Use products with both glucose and fructose, because glucose absorption alone saturates around sixty grams per hour and the second pathway is what lets you go higher. Practise at race intensity, not on easy rides, because a stomach that copes at an easy pace tells you nothing about one working at race effort.

For pacing: choose a ceiling in advance. If you have power, a percentage of your threshold that you do not exceed. If you have heart rate, a number you stay under for the first two hours regardless of feel. If you have neither, use breathing: you should be able to speak a full sentence comfortably for the entire first half of the bike.

The Bottom Line

People do not fail an Ironman because they were not fit enough. They fail because they ate a plan they had never tested and rode two hours faster than they should have.

Both are decided in training. Both are boring to fix. Both are worth more than any additional block of fitness you could build in the same time.