Moose, good for you for your ride! Keep it up. The cross-country thing is unrealistic at present, but your desire to help others is admirable. Do set realistic goals and respect your body's capabilities.
There are many ways to help charities as well as individuals in need in smaller ways. Look around you; you'll find some ways to make a positive difference in someone's life right now, while you train for your future cycling goals, if you wish. For instance, are there a few local students, maybe some who are not at all athletic at present, or who just need a good role model or mentor, who might like to begin cycling along with you? There are so many kids who need someone to guide, encourage, or simply show some genuine interest in them. (Only if you are suited in temperament and motivation for something like that, of course. Not everyone is or should.) Just one possibility to consider from a retired teacher. You don't have to make headlines to do a lot of good in the world.
BTW, I got a bicycle last year, and could not make it up one of the smaller hills in my neighborhood, even in the appropriately named granny gear, at first. NOT as easy as it looks!
Best wishes and God bless you in your endeavors! May you be an inspiration to others.