I watched my sister do Ironman Texas, and decided I was going to do it the very next year. I had not swam or rode a bike in probably 15 years.
Everyone told me to not get a tri-bike as my first bike. I am a very motivated person, and thought nothing was going to stop me from crossing the line in 11 months. I went to my local bike shop for a fit to see which tri-bike brands and models fit me best. They also cautioned me about getting a tri-bike first. Well, I ignored all the advice and got a Cervelo P2. I love that bike and rode it on a trainer a ton, and outside, and took it on several century rides leading up to Ironman Texas. Yes, I was the moron taking a tri-bike on grand fondo's. It was pretty uncomfortable riding on the horns for that distance, but I pushed through no problem.
It all worked out fine, and I just finished my 8th Ironman. My only regret was not buying a more expensive tri-bike with Di2 🙂 I've since bought a Cervelo S5 road bike, and I ride that one much more than my tri-bike outside.
If I had to do it over again, I would NOT have gotten a tri-bike first. I did not know what I was missing because the road bike is sooooo much more comfortable riding with groups and centuries. This is because it is unsafe to ride on your aerobars in groups.
One nice compromise (as others point out regarding putting aero bars on a road bike) is to get an aero road bike that has a more aggressive seat tube angle, such as a Cervelo S series. This will help you get pretty close to a tri-bike geometry when you add clip on aero bars.
But most important, go to a good bike shop and walk them through exactly what your goals and priorities are.
And good luck at IMTX in a few years. I'll be there every year. Just sign up for next year, and defer for free if you aren't ready 🙂 And if you love it, then save up to get a nice Di2 tri bike.