Running a marathon, especially if you're racing it, is a significant challenge. Twenty-six point two miles presents challenges that you'll never experience in shorter races.
Because of the distance, how you fuel before and during the race is critical. You can't store enough carbohydrates in your muscles, blood and liver beforehand for the whole distance.
The muscle damage you will inevitably experience in the final 10K of a marathon is an entirely new sensation. With tens of thousands of steps, likely on asphalt, you'll feel the damage for days after the race.
Finally, properly pacing during your marathon is much more important than it is during a shorter race. You can recover and still have a good race if you go out too fast in a 5K, but not in a marathon.
Even with all of these challenges that are unique to the marathon, the vast majority of runners will be able to finish the race. Despite low fuel, muscle damage and poor pacing, it's still possible to finish all 26.2 miles.
But when should you drop out of a marathon? What are the signs that point to a DNF ("Did Not Finish") as a good thing?
There are two important reasons that should compel you to drop out of a marathon.
1. You Sustain an Acute Injury
Every marathoner will experience aches and pains that are likely more significant than any felt during training or a tune-up race. These could cause you to doubt your fitness and ability to reach the finish line.
That's a normal part of running a marathon. You have to push past the full-body fatigue, brief muscle spasms, and growing sensation of being out of your comfort zone. A good rule to follow is that any pain should be dull or achy; those types of pain mean you're usually not doing any additional damage.