Where in KY are you? I'm NE of Louisville, and just started endurance myself last year. It is fun!
Check out Daniel Boone Distance Riders club - based in KY Welcome
There are several endurance rides in and near KY.
As for your horse, 7 is a great age to get started. Much better than starting too young! Getting started involves lots and lots of long slow distance. Be careful not to overdo it, especially in the beginning. Using a GPS will help you track mileage/speed and make training plans/goals. I use my Droid phone GPS app. I don't have a heart rate monitor- just learn how to check it manually yourself- but don't get too wrapped up in worrying about heart rate in the beginning anyway- you shouldn't be working your horse that hard until he has spent a long time on easy slower conditioning. Most horses get cardio fit much faster than their legs (tendons, ligaments, bone) get conditioned. Doing too much too fast is a good way to end up with a serious injury. That applies to all horses- not just endurance- but especially equine athletes.
There's a huge amount of info on the internet to read. Also a new book called Endurance 101 might be worth a read: Endurance 101 | a gentle guide to the sport of long-distance riding www.aerc.org
has a whole section about education. Endurance.Net
Hope to see you at some rides next year!