Please don't think of trail work as only just moseying down the path. For extended and rugged trail work you need strong bone and joints, hard hooves, a short back and thick neck, a powerful rear end, endurance, and a calm and steady, although willing, disposition...
I both agree and disagree with this. I'm not sure that a thick neck, powerful rear end and short back are necessary. I have ridden a narrow OTTB mare with a long back and long skinny neck on some very challenging, long trail rides with no problems. I have done mountain riding and endurance riding on many ottb's, and even our trails here down into the river valley involve steep hills, sketchy footing, river crossings and lots of thick brush, and my BO gets 5+ TB's off the track every year that we retrain for trails in that terrain.
Also my arab, who is the best trail horse I've ever owned, can get hot, has a short, skinny neck and no hind end. Powerful is not a word I would use to describe her. Infact, I joke all the time that she is like riding a wet noodle, narrow and flexible
. That said, she is observant, can go forever, is very sure footed and goes up, down or over whatever you point her at.
I think a good trail horse has to be a horse with a good head on its shoulders that thinks things through, is sure footed, obedient and especially sound, sound, sound. Lots of thoroughbreds fit that criteria.