Endurance is pretty serious business. It's tough on your horse... Some of them just flop down where they stand from exhaustion, dehydration, colic... Some just flat out flop to the ground and die. If you finish
the run, you are considered successful. To heck with winning.
It takes about six months of conditioning 5 or six days a week to be really prepared for a 25 mile step-up run. We won't even get into 50 and 100 mile races.
At Lacey's "advanced age", I don't think endurance would be good for her. It would be too stressful on her system, IMO. CTR, however, would be fine. Actually, I think it would be a great thing for you and Lacey to try out.
It's mostly walking and trotting, with frequently vet checks. It's pretty hard to get a sick horse through. Conditioning will take about 4-6 months, and most runs are 15-20 miles a day if you're running novice or competitive pleasure. Opened riders can go 25+ miles a day. You can choose to either ride one day of 15 or so miles or two days, coming up with a two day total of around 30 miles.
In CT, horse and rider are judged separately. Your horse is judged on condition and you are judged on your horsemanship. Both you and your horse start out with scores of 100 and it goes down from there. The highest score of your division at the end of the day wins.
Horsemanship means how well you ride (going up and down hills without hindering your horse, negotiating obstacles) and how well you take care of your horse (stabling, well fitting tack, etc.).
Where are you located? CTR is divided into five division based on geographic location. I'm in division five. Ha ha, maybe we could ride together!