Or if I misunderstood you and you're asking about your first day trip, start with 35-40km (up to about 25mi.) That's enough to be a bit of a challenge and maybe leave you a bit sore at the end, but should be do-able after a few weeks riding.
Bring enough water to allow 1 bottle per hour (eg. if there is a shop or water at the other end you don't need to carry 2-3 bottles all the way), and a snack eg. sandwich or muesli bar or two.
Sunscreen.
Pump, spare tube, tyre levers, tyre patch kit.
Bike lock so you can leave it for a while.
Mobile phone.
That should do it!