Well, I'll answer my own question.
I do it because for some reason I'm motivated by challenging goals. NOt small easily attained goals, but goals that are *big*, demanding, unique, and punishing. I have no idea why this is, but it's true. On the enneagram I'm an 8 (I do believe this has something to do with it). If you don't know what I'm talking about go to
www.enneagraminstitute.com and take the free profile. Then, read more about yourself under traditional enneagram. As a lover of astrology, personality profiles, etc this one floored me (but I digress).
The reality is that if I could run forever with out getting hurt I would do it. but, since I'm trying to be smarter and not put myself totally in injury's way, tris are a terrific thing to do. The only event that I hadn't done before was the bike, and I really have come to love it. Although I didn't predict it, swimming can be daunting because I burn out at the pool. but, overall, tris are liberating, theraputic, and a great thing to pour myself into. It allows me to be as intense, vengeful, narotic, obsessive, or competitive as I can be without complaints. It's perfect! LOL!