You may have noticed that I didn’t tell you how many miles to travel each week in your training. I recommend that you stick to “time” rather than “distance” at first. When you are riding your horse at a mixture of walk and trot for two hours or more, borrow a GPS (or a bicycle with an odometer) to find out how far you are travelling in your average workout. If you are covering 10 miles or more in 2 hours, your pace is adequate for an endurance ride. If you are covering less than 10 miles in 2 hours, add more trot work or practice trotting a little faster.
When you can do ten miles in two hours twice a week, you're ready to start a 25-mile limited distance ride. You probably won't be first across the finish line at that pace, but your horse will be fit to do the distance, and as you know, the motto of the sport is "To Finish is to Win."
Remember that the person who finishes the ride in 11th place gets the same prize and number of milage points as the person who finishes dead last...so take it easy and slowly the first year, in order to build up a horse who will be able to finish and enjoy his events during the second year and beyond.
When my grand ma was 92
She did PT better than you