@ascentale @edd @bikenite I used to do these fairly often. I had a Jansport backpack that included a CamelBak system and that carried everything I needed. A bladder of hydration solution, extra sunscreen, my phone, baggies of dried bananas and nuts, money, and a lightweight bike lock.
The under seat bag had stuff for a quick tube change. I now ride tubeless so that's less important.
I'd stuff some extra energy snacks in my jersey pockets.
If possible, plan real stops for rest and food.