I have competed on TBs for over 20 years. While 13 is older, it's not too old. May horses compete into their 20's. Rio Grand Sultan took the World championship at age 21. My TB got 8th place last month and she is 20.
The trails do vary a lot. Go online and ask people what a particular ride's trails are like.
For information, there are a lot of great books out there. Ridecamp at endurance.net is a great forum. Volunteering at a few rides before is invaluable. Crewing for someone at a ride helps.
Your timeline, start next spring , is great. That gives you enough time to condition him correctly. I always give my horses 6 months of conditioning before their 1st 50. If he has never camped, start doing some pleasure camping/riding with him.