For IRONMAN or IRONMAN 70.3 racing, you have two types of shoes to choose from: road shoes or triathlon-specific cycling shoes. There are pros and cons to each, but I still recommend a road shoe for long course racing.
Road shoes are stiffer (better power transfer), more comfortable and they will feel more secure when they’re on, especially over 112 miles.
The only downside to road shoes is that they don’t have the simple “one-strap” quick entry/exit system that triathlon shoes provide. However, you can find a pair of road shoes with a simple BOA system and with practice, you will be able to get in and out of them quickly. Over an IRONMAN or IRONMAN 70.3, comfort outweighs a minimally slower transition. Additionally, after a couple of hours on the bike, the one strap closure system you find on many triathlon shoes may not feel as secure as a road-shoe closure system. So, that minimal transition time difference may be a wash.
What About Pedals?
A road pedal is going to have a bigger platform when we compare it to a MTB-style pedal. This bigger platform is going to reduce the risk of hot spots over a long ride and be slightly more efficient than a MTB-style pedal. For this reason, I recommend a road cycling shoe with a road-specific cleat for IRONMAN racing.