You have to be able to go faster in training, for a shorter time period, than you will ever go in a race. I know way too many IM distance people who never push the top end/intensity, so they never improve past the pace they do their endurance training at (e.g., 22 mph on the bike).

Kiel Reinen (probably spelled wrong, but he rides for Trek-Segfredo) wrote an interesting article about his jump from domestic racing to the World Tour. His 1,5, and 20 minutes power numbers weren't changing, it was just that he could hit the same numbers after 4-5 hours at riding. The point being that you need to be able to hit close to your best FTP before you worry about doing it for 5 hours.

My guess is that there is something between a reverse periodization and typical build that probably would work well for distance athletes who have a life. Maybe rest week, intensity week, mixed week and an endurance week, rest week,...