How do you describe an Ironman to someone who doesn't know?

"Describe an Ironman" could mean "a person who is an Ironman" as my learned colleagues above have done,
or,
it could mean "a test of endurance conducted from point A to point B covering 2.4miles of swimming followed by 112 miles of bicycling, followed immediately by 26.2 miles on foot, done in the fastest possible time usually involving at least 2,000 competitors."

That may sound dull but it is the most economical and accurate definition I can compose.

Libor calls it a "sufferfest."

Metaphorically it might be something like this:

Ironman is a test of courage stressing the body and emotions to their limits by combining swimming, biking and running performed at optimum speed against, and with, about 2,000 like-minded souls. It requires commitment and fortitude, manifests a primitive need to push yourself as close to exhaustion as you dare in an exploration of your own character. It expresses the human need to stand on the edge of the deep and peer into the abyss just far enough that you can then draw back and scream defiance in the face of death, then to return, trembling, to the quotidian with renewed vigour and the confidence that comes from knowing that you can not only survive, but that you can prevail.