You’ll be fine. With a Gran Fondo, you won’t be racing the whole time as only the timed segments count. You can either go hard on the timed segments or take it leisurely like a century ride.
I’m doing the Gran route but anyone can join for any distance. The fun thing about Gran Fondo is that you can sort of ride together in between segments so we can all ride together until it splits or at least meet at the start and the first aid station.
This is a pretty hilly ride so would recommend either using a road bike or changing the cassette for hills otherwise it’s going to be a long day!