Ok Kid....... If the grumps and frumps didn't chase you off, I'll throw out some gray beard wisdom (actually I only got back to riding 3-4 years ago, but I do have a grey beard).
Chances are that you and your bike would make it a hundred miles if you had to go right now. The bikes bearings might be toast and you might be walking funny for a while. Or you might end up broken down 70 miles from where you want to be because any number of things went wrong. So with a little luck you can be the kid that barely survived a 100 mile ride. Or with a bit of prep work and training you might make it look easy.

First thing I would do, if possible, is find a bike co-op. Co-op have parts tools an folks who want to help kids get on bikes. If you know how to fix everything on you bike, and have gone over everything, the less you will be worried about something exploding's at mile 99.5. Working with a Co-op will also be a lot cheaper than a bike shop, but your still going to end up putting another $100+ into an old bike getting it ready to ride another 1000 miles.

The other 900 miles are training. I'm not a juvenile sports doctor, but I've seen kids get hurt with to much too fast in school sports. At least for my knees its important to build up mileage slowly. If you haven't been riding much a couple miles every other day, riding a little farther every time, and adjusting the bike fit as you go will help keep you from trashing you knees . 100 miles is a long time to do something that hurts you 1.3 times per second. Even though it feels good to mash on high gears, its better to be spinning the cranks in a gear that feels too low while training. A rule of thumb that I read on the internet is that if you can easily ride a hundred miles spread out during a week, your ready for a century. I haven't done a century in one shot , but have done a 300 mile week loaded touring, after building up to a regular 100 mile week. I didn't die while having fun. Good luck on your adventure.