You'll murder it - start riding now, get some miles in your legs and rest will follow. You're young, don't fixate on diet or fitness, there's no substitute for being out there and just doing it.
Once you've got some fitness & confidence you can start thinking about the trip itself, but the reality of a charity ride / sportive is there will be plenty of people in the same position as you - there will be loads of refreshment stops along the way so you can always stop & have a breather - nobody is timing you, it's not the tour de france.
That first step (pedal stroke?) is always the most daunting but you CAN do it