Hawkear,
I've done one...the Adventure Triathlon in Maxwell, Nebraska. I posted my race story on my team's triathlon website if you want to read it.
http://www.teamnebraska.org/RaceStor...hlonStory.html
It was my second tri; however, I did alot of mountain biking when I lived in Arizona. My biggest suggestion would be to swim in open water. It was alot different than swimming in the pool. It was the hardest tri I've done but the most rewarding. I really enjoyed it. Good luck. I'm looking forward to hearing how it goes for you.