I have to agree with the others who point out that most people who are really interested in endurance will not be looking at a QH “prospect”. That doesn’t mean that particular horse can’t do it, but “prospect” means nothing really and traditionally QH’s aren’t the “go to” breed for the sport. So serious riders aren’t going to look at her when there are $500-1000 Arab “prospects” all over the place.
However a well-trained, well-mannered, level-headed QH trail horse is VERY marketable, since far more people do trails than endurance.
IF the mare is all that, than market her as such and drop the endurance angle; you won’t have to sell her cheap.
So in other words: she doesn’t freak out often at all the stuff we see on the trails, she does not jig or act stupid when asked to simply WALK, she ties, trailers willingly and stands calmly when being mounted, she crosses water, small logs, etc. without having major conniptions, and she is sound enough (and has good feet) to do the job.
THAT is a trail horse; if she can’t do all of that, she is just a “trail horse prospect” and you are back to square one.