Road or tri bike for ironman?

Thinking of trying for my first IM in Arizona. It is a totally flat course, so I am wondering how much energy is saved on a tri bike. I have a great road bike, and the distance and time on the bike is not a problem for me, but I think I will struggle on the run so am wondering if it would make sense to rent or get a tri bike to save energy for the run. I can also get some tri bars for my road bike and maybe that gets me 90% there?