First a digression to note that I previously posted on Mencken, here, with this lovely and tantalizing quote on the Unknowable:
Penetrating so many secrets, we cease to believe in the unknowable. But there it sits, nevertheless, calmly licking its chops.
At times all of us get casual and complacent in our running, and when in the course of going some far distance, the unexpected sometimes happens: we approach or cross "the edge" and into the unknowable. It's a strange and wondrous experience indeed. For there we learn about who we really are, by breathing in a zone that the sedentary never even know exists.
Anyway, digression closed, and I want to come back to the question at hand about Mencken's nailing it about why I am an Ultrarunner (a term that probably did not even exist back then). From a Pharyngula post, here:
You ask me, in brief, what satisfaction I get out of life, and why I go on working. I go on working for the same reason that a hen goes on laying eggs. There is in every living creature an obscure but powerful impulse to active functioning. Life demands to be lived. Inaction, save as a measure of recuperation between bursts of activity, is painful and dangerous to the healthy organism-in fact, it is almost impossible. Only the dying can be really idle.
The precise form of an individual's activity is determined, of course, by the equipment with which he came into the world. In other words, it is determined by his heredity. I do not lay eggs, as a hen does, because I was born without any equipment for it. For the same reason I do not get myself elected to Congress, or play the violoncello, or teach metaphysics in a college, or work in a steel mill. What I do is simply what lies easiest to my hand. It happens that I was born with an intense and insatiable interest in ideas, and thus like to play with them. It happens also that I was born with rather more than the average facility for putting them into words. In consequence, I am a writer and editor, which is to say, a dealer in them and concoctor of them.
To paraphrase, it also happens that I must have been born an Ultrarunner...and was lucky enough to discover that fact and act upon it.