Good bone and hoof are essential to long term success. Once you determine your horse can do the job, take a good look at your tack. All types of saddles are used in competitive trail, but it is pertinent that it fits your horse very well. You won't get far with a sore back. I let my horses live out doors with access to their stalls day and night.
Most pasture horses are healthier, stronger, and more fit than a stall kept horse.
Many riders keep their horses stalled either during the day or at night which seems to be a good system as well. Some horses are just not cut out to trot down the trail for hours, and these individuals will probably let you know this in the first couple of months of training. At the start of your program, work with your veterinarian and farrier. Have a normal blood count done, as this will help you with any problem solving later. Have his teeth floated, as well as vaccinations and de-worming done.