Most AERC Rides have age restrictions of 8. The rider must be at least 8 years of age, and the horse must be at least 4 years for limited distance rides (25-30 miles), or the horse must be at least 5 years for regular endurance rides (50+ miles).
Training wise, I usually take my horse out for 2-4 hours, 3 days a week at an almost constant extended trot with some cantering/galloping where I can and walking where I need to. Make sure the horses have at least 1 day of recovery time after training rides- the muscle needs time to rebuild after such a work out! We are from PA, so we go up to the local skiing mountain and run up that for preparation for the more mountainous rides we go to. When training, I ride in all the tack I will be using at the race, so my horse is used to the weight. I like to time my rides, since I know the mileage of all the trails I use. I also check my horse's pulse, respiration, hydration, ect when I come home (a mach vet check, if you will) just to see where they are at. :)
Where are you located? What distance where you planning on doing? This is definitely not for the faint of heart! Have fun hitting the trails with your horse!