I've never ridden it myself, but here's something from the FAQ:
Q: Any suggestions for training before BAK?
A: One suggestion is that you should be able to do 30 miles in 3 hours or less. Make sure you can ride 50-70 miles in a day--don't wait to find out when you're on BAK. A good way to get in shape is to ride with a local bicycle club. You'll be able to put in your miles with other cyclists and learn how to ride safely within a group.
http://www.bak.org/about/faq.html
That might be a little tough for a 9 year old on fat tires? I'm not discouraging you, but it's a long ride. My advice: but the nicest most padded shorts you can afford. Going the distance might not be too terribly hard, but sitting on your rear for that long is!
