I have a 1980's Nishiki (I think) bike that I got from my dad. I've had it for 10 years, I'm not sure how long he had it. I could ride all day on it, but I didn't get anywhere fast. I was estimating 7.5 hours for 112 mile bike ride. Obviously when you only get 16 hours, that's a huge chunk, not to mention the physical effort.
I added clipless pedals, a better seat, aerobars, and new wheels and I was able to cut that time down by 30 minutes.
Then I tried out a random triathlon bike and that time was around 6.5 hours.
You can literally buy down your time when it comes to gear for bicycles in the IRONMAN!
I'm all for cheap and using what you have, but man, that hour makes a massive difference. I'm attempting to avoid the temptations of spending thousands right now to buy time, but I hope to space it out over a few years so that I don't go poor.