If I could only keep one session a week, it wouldn't be the hardest one. It'd be the long bike.
Why the long ride wins
Not because it destroys me the way VO2 max intervals do. Because Ironman isn't really about how hard you can go, it's about durability — and nothing trains that quite like hours in the saddle.
It's time in position, which matters more the longer the race gets. It's nutrition, actually practising eating and drinking while moving instead of guessing on race day. It's aerobic fitness, built the boring way, over hours instead of minutes. And it's mental endurance, which you can't fake in a forty-minute session.
One session, loads of adaptations stacked on top of each other. If I had to delete every other session and keep just one, this is it. What would you keep?