Popular ones attract 20,000 competitors. In ultra running the entrants are still counted in dozens. They tend to be older runners, in their 30's and 40's, who have been at the sport for years. They don't all run 100 miles a week, though many do, but they train consistently and are generally veteran marathoners. Maybe it's a feeling you can endure a little longer testing your limits.
Most ultra runners say doing longer distances is easier on the body than shorter distances.
Ultras produce fatigue, not muscle soreness, they say. But most people don't believe it. It's kind of fun to have people think of you as a little crazy. Still, the pain does come, for those who make miscalculations and went out too fast, for those who overestimated their capabilities. There are always times in an ultra when the body must be willed onward.