I really appreciate the active discussion going on here! I'm not going to offer my responses just yet since I'm still sorting everything out and decided where I sit (pun intended lol), hence why I asked the question in the first place.
To offer some information about the demands placed on an endurance rider: Basically, we do what works. Your riding style isn't judged, so there's no real consistency. Plus, anyone can up and try it, so you get all types of saddles, gear, experiences, styles....etc.
Here's what IS consistent:
Your horse is mostly trotting, and, especially in my case, the horse is moving pretty big. Some level of posting/standing is necessary to keep your weight from coming down of your horse's back and to stay balanced as well (it's impossible to sit my horse's trot, nor should I be sitting it). As mentioned, standing for 50 miles would be nearly impossible, so posting is what I prefer to do over most of our miles, though I do stand occasionally either to stretch my legs a little or when we're doing a fast stretch (extended trot - I sit the canter) so that I can free him up to move (though I still need to be balanced so he can move well). Keeping the horse's back from getting sore is the goal, so I need to be as balanced as possible and do my job to make it easier for my horse to do his job over the many miles we cover.
I have a Big Horn endurance saddle, model 117, in case anyone wants to take that into account. However, my primary goal is to sit correctly, and if this ends up putting me in the wrong position, I would rather sell it and get a more fitting saddle than try to fight it or sit incorrectly.