Jitterbug is older but in very good shape. Would he be suitable for competitive trail riding?
I do competitive trail under UMECRA rules, which is basically endurance since we get even LESS time to complete our miles than endurance riders do (5 hrs, 25 minutes for a 25 miler vs. 6 hours for endurance riders). Ohio comp riders always make jokes about the Michigan rides.
If he's in very good shape, you could probably do a 15 mile novice ride to start. It will give you an idea of how he will handle longer distances and you can see how the more experienced riders "ride the rides".
Unlike endurance, there is no "national" organization for competitive trail rides. There is the new ACTA (American Competitive Trail Assoc.) but they are fairly new on the scene and don't offer many rides yet. Also, because I am used to very fast-paced rides, I would probably be bored witless at one of their slower, obstacle based rides. NATRC (North American Trail Ride Conference) mostly sanctions rides out west and in the south. ECTRA (Eastern Competitive Trail Ride Assoc) does the east coast. SEDRA (South Eastern Distance Riding Assoc) does the southeast. Middle of the Trail (MOTDRA) does the central US (TX, KS, MO, OK). UMECRA (Upper Midwest Endurance and Competitive Rides Assoc) does the upper midwest/Great Lakes. OAATS (Ohio Arabian and All-breed Trail System) does Ohio. And unfortanately each one has their own set of rules.
If you are riding your horse for 1 hr+ 4 times a week or so, he could probably handle the shorter distances 15-25 just fine, although I would recommend starting with a 12-15 mile novice ride. Do you go up and down a lot of hills? Through deep sand? Can he go through rivers? Navigate switchbacks? Make sure he's familiar with the type of terrain he will encounter in the competition. The AERC hosted their 50 mile champ ride in Michigan one year. A lot of out of state riders scoffed, because there were no "real hills" in central Michigan. Nope, but there's a lot of DEEP SAND. A lot of horses were pulled because they were pulling tendons, getting rubbed raw because sand got under splint boots and for metabolic reasons (running through deep sand takes a toll on your cardiovascular sustem). In reverse, Michigan riders have problems in southern Ohio because of the sticky clay there. The Arabian Horse Association champ ride was in Florida one year, a lot of horses from the northern states pulled due to humidity.
The best way to condition for long distance riding is LSD (long, slow distance) by just doing lots of walking. Once the base is established, you can start working on speed and cardiovascular system.
I've been distance riding since 2000. If you have any questions, feel free to ask.