Yes, their level of fitness is something else. And that .80 If for 5 hours, comes after over an hour of swimming and before going running for 3 hours. No way I could manage that. I'm tempted to try a triathlon, but the local ones here are 20,000 Yen, and my swimming is very rusty, and the amount of hours it would take me to get it into shape, doesn't appeal. I think the only way that is going to happen is if the little one starts wanting to go to the pool.
Yes, CP is higher than FTP so it will alter the numbers a bit, but to quote Joe Friel, "The men’s pro winner in an Ironman almost always finishes the ride with an IF of about 80%. You can count on it." But obviously, top pros are on the course for less time, so they can target a higher IF than an age grouper who will be out there for a lot longer. I think the goal is more the overall TSS that the ride produces, rather than the actual IF, so the quicker you can go, the harder you can go, within reason.