My first ever triathlon was the Ironman european championships in 2001. It came about over a few beers at christmas time (lots of things seem a good idea after a few beers!!) I was in the Royal Air Force and my boss at the time suggested doing a tri, he had been pretty good in his younger years and a dab hand at swimming 10K's!! I could manage 4 lengths (thats 100m).
When I got to work the following monday he had told me that I had been entered for a triathlon and that i had better get some training done. This gave me 9 months to go from rugby player to Ironman, how naivety is bliss!
Anyway, i set to work training and having the odd beer or three (well it was in the summer time!). So the first time i had ever swam over one mile was on the day of the race and the only time i had ever swam in open water was the day before IM! I finished the race in about 13 hrs 12 mins and was happy to complete the event and proceed to eat for england for about a fornight.
I guess the moral of the story is that you have to keep things in perspective, IM is achievable, you can still have fun in the build up and that naivety is bliss!
good luck, i hope this helps? any more info let me know!