Ironman and a 100 Mile Trail Run are both the toughest non-obscure events in their arenas. Yes, there are ultra triathlons, and trail runs longer than 100 miles but they are rare, and not considered for this discussion.
Now that I have done both, I have been asked "Which one is tougher?"
If I gave you a simple answer, this wouldn't be much of a blog report. Let's look at it from different perspectives.
In terms of of training, I found Ironman tougher. When training for the 100 miler, I ran averaged 70+ miles per week, building up and peaking at 100 miles per week. I did several 30 to 35 mile training runs, and one 50 mile trail run. When training for Ironman, I maintained my same running training, but added some serious swim and bike training. I still did the Saturday 30+ mile training run, but went for a 2.5 mile swim afterwards. My typical midweek 17 mile trail run was done before or after a 35 mile bike ride. My Sunday hilly 15 mile run was done after biking 75 to 100 miles. Some days I did all three disciplines. Most other days I did two.
Complexity, and Planning/Organization
Ironman is more challenging than running a 100 miles in terms of complexity and planning / organization, though its closer than most would think. Obviously, more equipment is needed for Ironman: wet suit, goggles, swim cap, bike, helmet, bike shoes, running shoes. However, you know exactly where/when you will need them. In a hundred miler, you'll need flashlights and extra batteries for the night portion of your run, sunglasses and hat for the day time, and changes of clothes/shoes for a wider variety of weather conditions, but you'll need to plan and figure out when you'll need them. Drop bag locations can be 10 to 25 miles apart. You need to know when you'll get where, and what items you'll need. If you end up running slower than planned, and put your flashlights in the bag you thought you'd get to at 8pm (before dark), and you are still 10 miles away when darkness comes, you are sunk. When running partner Dennis and I did the 100 miler, we changed into dry shoes at mile 50, after the light drizzle stopped, only to be hit with torrential downpours 30 minutes later. We had not packed dry shoes in the bags we had for later aid stations.
Endurance events require mental effort as well as physical effort. I found Ironman to be more of a mental challenge than the 100 miler, however, I did not run the 100 miler alone. Running partner Dennis and I stayed together the entire time. Many 100 milers allow pacers for the later stages of the race to provide company and moral support. This category its close, but I'd say Ironman is tougher mentally.
Not only are these events physically and mentally challenging, there is a cerebral component as well. I am referring to how much you need to pay attention to what you are doing. For example, a run on a low traffic paved bike trail, there is very little cerebral effort. You can set you legs on auto-pilot, and zone out.
During the swim and bike portions of Ironman, there is a lot of cerebral effort. You constantly need to be alert to your surroundings. In the swim, you need to be watching out for flailing limbs. During the bike, you need to be aware of how close you are to other competitors, as well as constantly watching the road for bumps, debris or hazards. During the run portion of Ironman, the cerebral effort is relatively low. You still need to be paying attention to what you are doing, but not high alert level needed during bike and swim.
During a trail run, when the footing is technical (which means lots of rocks and roots, and bumps and hazards), cerebral effort is pretty high. You need to constantly scan the trail ahead of you, looking for exactly the right place your foot should come down. Coming down on a poorly placed foot can result in a twisted ankle, a fall, or a body jolt that kills your momentum.
For cerebral effort, its close, but I think I have to say the 100 miler is tougher.
Absolutely, no question, not even close. The physical effort for a 100 mile trail run is much harder than an Ironman. If you ranked physical effort on a scale of 1 to 10, I'd say the effort level during Ironman's swim was about a 6. The bike fluctuated from 3 on downhills, 5 on flat sections, 7 to 9 on uphills, and spiked to 10 on the killer hills. The run portion of the marathon would rank around 8 to 9.
During the 100 miler, the effort level starts at about 6, gradually climbs to 9 or 10 at mile 75, and stays at 9 or 10 for the next 15 to 20 miles, and finishes with solid 10 for the effort level. Even though we were walking most of the last 25 miles, we were excruciatingly fatigued, and it took every thing we had to keep moving forward for the last 7 miles.
Statistics and Tidbits
Ironman events have a time limit of 17 hours. Mine took 12 hours and 13 minutes. The 100 miler I did had a time limit of 30 hours. Mine took 29 hours and 7 minutes.
In both events, I got to a point where I had marathon between me and the finish. During Ironman, that occurred around 3:15 in the afternoon. During the 100 miler, that happened around 3:15 in the morning.
There were eight Ironman events in the United States in the past year with a total of 17183 starters and 15993 finishers, yielding a 92% finish rate.
I counted sixty three 100 mile trail runs in the US in the past year, with 3971 finishers. Only 34 of the events listed the number of starters. Among the 34 events that listed the number of starters, the average finish rate was 61%.
One 100 miler, The Barkley in Tennessee, had 40 starters and 1 finisher.
When Dennis and I ran our 100 miler and met up with other runners, we'd often ask how many 100 milers they had done. Nearly every person gave us 2 numbers: the number they had started and the number they had finished.
If a reasonably healthy non-runner adult wanted to do either event...
I would define reasonably healthy as someone who can walk 2 miles in 30 minutes, can swim for 100 yards in about 3 minutes, and bike 12 miles in an hour, with full rest in between each.
If a reasonably healthy non-runner adult wanted to do an Ironman, I would estimate it would take about 2 to 3 years of consistent training to build up to an Ironman.
If a reasonably healthy non-runner adult wanted to do a 100 miler, I would estimate it would take 3 to 4 years of consistent training to build up to a 100 mile trail run.
In August of 1999, I was a 'reasonably healthy non-runner adult'.
Endurance events simply require a substantial fitness base. Building this base requires pure and simple hard work.
Both of these events are extremely tough. As someone who has done one of each, I can say that success in one is absolutely zero guarantee of being able to complete the other. Though both require years of dedication and hours of hard work, no matter what your experience level is, there is no guarantee of being able to finish either event.