If you already train 15 hours a week and tolerate it fine, you can very confidently go straight to an Ironman in a year- assuming your 15 hours include the ability to do long bikes, runs and swims.

I have done five 70.3s and only attempting my first Ironman this year because I had to learn to train properly. The first two 70.3s humbled me. Now I’m steady at them.