Whereas the extreme sports-types go for the powerful adrenalin rush, endurance athletes are more into the soothing endorphin-like quality of the runner's high. It is a powerful, yet very peaceful high with a deeply satisfying somatic quality. That sets it apart from the feel-good bouts one gets from successfully controlling a device, as in driving a race-car or flying a plane. The endurance high is felt deeply in one's bones, ligaments, and muscles.
I have found that endurance athletes do not really know, or want to know, or cannot express why they are doing what they do. Some will state bluntly that it never occurred to them to ask. As many would say:"If you have to ask, then you won't understand the answer."
In an interview, former pro triathlete Ken Glah said he started competing in Ironman because he really liked training for it. Most triathletes find routine hard exercise relaxing, soothing and invigorating. They often say it sets their mind free.
If you look at the competitor lists you will see that many also engage in endurance sports as a way to overcome or deal with adverse events in their lives, or the lives of relatives and friends. The World Triathlon Corporation (WTC), owner of Ironman often promotes and highlights these feats. People battling deadly diseases or those with relatives suffering from deadly diseases, often engage in endurance events.
Endurance appeals to older competitors who tend to do better, relatively speaking in long events. The longer the event, the more it relies on strength and endurance versus raw speed and peak performances. That favors more mature individuals over energetic teens.
While many no doubt joined the ranks in pursuit of a health benefit or in the hope of delaying the ravages of aging, it is often not the main reason. Unless people find a way to enjoy the distance, they usually do not last all that long in this sport. Endurance athletes are in for it because they seem to need it for what it is. There is that funny addictive quality again, and there is no denying that it plays a much greater role than anyone is willing to admit.
Although cameraderie is an essential component, endurance racing is largely about the "loneliness of the distance runner." Just you and your mind and the long road ahead. As many have pointed out, the athlete is often his or her's only real opponent, and nowhere is that more clear than in an endurance event.