I got written on today as part of my Very First Triathlon! (I'm either Racer #29 and 119 years old, or vice versa.)
It was a sprint distance triathlon, with a 500 yard swim, 11 mile bike ride, and 3 mile run. I'm still learning how to do the running thing successfully so I walked parts of the run segment, but I finished! And I didn't just limp across the finish, I had a solid (if not blazingly fast) time. So now I know I can do this and I have a time benchmarked which I can try to improve during subsequent triathlons. I definitely want to do more - I can see how this can be very addicting (isn't self-confidence wonderful?). Maybe if I manage to get myself into better shape I'll be able to do an olympic distance one as well.