It's almost counter-intuitive, but I was a lot more exhausted after the half-Ironman than after the Ironman. I pushed myself a lot harder during the swim and especially the bike leg at the half-distance race. At the full race I was more easy-going, making sure I'd finish.
http://en.wikipedia.org/wiki/Triathlon#Standard_race_distanc...
I figure its the perfect length to get into it - each individual section I could do right now, but the combined effort will be killer :)