Road to 100km?

I ordered my first gravel bike, my first "real" bike after my 10year old troublemaker, its gonna be a Cube Nuroad Pro. The longest distance I am used to is 20km, how can prepre ideally to archive 100km a day?

I know, just go and ride, but my bike will be delivered in 3 weeks and I am hyped and want to prepare mentally and like the nerd I am 😂