Unequivocally, Absolutely, HELL NO!!! That is a total myth. I can safely say I know more people that completed their first marathon during an Ironman, than those with marathon experience. A normal marathon and an Ironman marathon only have one thing in common, 26.2 miles.
Let me tell you why they are completely different:
Reason 1) The bike. The 112 mile bike ride before the 26.2 mile run makes all the difference in the world. Ironman distance races are NOT about the run, they’re about the bike. How you train for the bike, how conditioned you are for the bike, how much fluid you took in on the bike, how many calories you took in on the bike, what your pacing was on the bike, etc… all prepare you for the marathon. You can be a 2:30 marathoner, but if you expend too much energy on the bike or don’t master your nutrition, you will be walking the marathon.
Reason 2) The mindset. The marathon in a Ironman can be one of the hardest things in sports mentally. By the time you get to the run on an Ironman you are between 7 and 10 hours into your race, and most are NOT thinking about crushing the marathon…..they are thinking about how bad my ass hurts, how I have to go to the bathroom, how stiff my legs are, how the hell am I going to get through this 26.2 mile run, etc… Your mindset is checking heartrate, pace, and getting to the next mile or two. In a normal marathon, most people are banking time and thinking about the upcoming 20-mile wall, and it is usually higher rate of perceived exertion.
Reason 3) Your training. Running 26.2 miles is brutal on the body. You run the risk of injury, and can take weeks to recover from the pounding your legs. Recovering from that marathon could literally ruin your Ironman training. Those weeks are better spent building a proper base or building speed. PLEASE DO NOT RUN A MARATHON IN THE MIDDLE OF YOUR IRONMAN TRAINING. If you are terrified of the marathon then incorporate back to back running days of 10-15 miles at a slow easy pace, which should help you with your confidence. Just make sure you have a rest week planned immediately after the back to back longer run.
Obviously, there are exceptions to the rule, but most of the population will only be negatively effected by it. You may run a marathon 7+ months before the Ironman (this is still a risk of injury), and even then I suggest a slow easy pace. You could better served doing some strength training 7 months before Ironman, base training, and focusing on your technique.
Bottom line. You do not need a run a marathon before you do an Ironman.
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