FTP is not the power you can hold for a hour. This is the defintion that gets oft-repeated. The actual concept is that it's the power you can hold for a long time, in the range of 40-90 min say depending on your physiology. We're all different.

When I'm trained up to endurance efforts, I can ride at 75-77% FTP for 5-6 hours. I can sometimes knock out a 2 hour group ride at 88-90% FTP. I've done 100% for 70 minutes.

Part of the answer to this is knowing/learning what you can get away with based on how resistant to fatigue you are. I usually ignore the adage of not going out too hard, I tend to go out hard and see how it goes. Sometimes I pay the price at the end, sometimes I have a great day. But so far I never fail to get it done on a long ride, even if I go out too hard.

I'd say do test rides and discover what you can do prior to your event.