Although I 100% agree with the others that you should not have an endurance race as a goal for this horse, you can and should have a conditioning plan in place to keep her in good health.
1st importance is footing for this mare, not too firm and not too soft, and possible hoof boots would help to cushion her joints. Also Forta-flex or other joint medicine.
2nd, I usually focus on improving a horse's lungs through interval training. Periods of walk & trot, to begin, then working up to w/t/c sets. Sart with something like 10 steps walk, two strides of trot, repeat. This can be done lunging or riding, depending on the horse's current condition.
Last is to build the muscles through conditioning. Shortenings & lengthening in each gait, longer periods within each gait, up & down hills, etc.
Check heart rate & recovery time religiously.
work 3 days, one day off, repeat.
Keep assessing her tolerance as you go along, and she should make a very nice trail horse. I would not plan on trying to be competative if you choose to try one of those 10 mile rides. Focus instead on keeping the mare sound.