As
GhostRider62 said, we don't have enough info.
But coasting for 30% of the time on gravel sounds extreme. Except for descents, you can't coast for extended periods of time on gravel b/c the rough surface eats all of your momentum and forces you to add more juice.
You can clarify some of what might be going on by giving us some more information. Is your general technique something like pedal-pedal-pedal-coast-pedal-pedal-coast? If that's the case, you need to work on your technique. Apart from hills, turns, and momentary bike handling maneuvers, you basically never want to stop pedaling.
Doing the Unbound 100 is a heck of a way to begin with gravel centuries.