I'll type some stuff, but I'm not a trainer or very good rider and I don't do any horse sports and cannot IMAGINE riding 50 miles at a shot, so take it with a big steaming helping of FWIW:
1 - We had a little Arabian mare who was broken for us, and then I rode her out. We sold her a couple of years ago, but I rode her a lot when she was very green. It worked OK because she was a good-natured and forgiving mare.
Anyways, when I first started riding her 750 lbs with my 180 lbs + 30 lb saddle, she rode head high the whole time. Hmmm - 28% of her weight and an inexperienced rider who bounced too much. Could I blame her for hollowing her back?
So I started riding her in two point, even in my western saddle. The horse's hindquarters powers up thru the loin and eventually hits the shoulder, but the withers is like the fulcrum point. It is the place were the back moves the least when functioning correctly. The change in her movement was almost immediate. The saddle was free to pivot around the point where the stirrups attach, and my inexperienced butt wasn't bouncing on her loins. Once she started using her back, she began to understand and use it even when my weight was in the saddle. But she needed to get stronger to move her back with my weight, and she had no incentive to do that while I was putting pressure on her loins and back.
2 - Tried it again with our Appy/Arab and cantering. We learned cantering together. I'm sure he had done it, but he had gone several years without cantering when I first asked him to on MY first attempt at cantering. I was trying to 'sit the canter' like a person might sit the trot, and I ended up flying in loose formation with him. The picture below was taken about 30 seconds into our first attempt, and is embarrassing:
Doesn't get much uglier than that!
It was enlightening to realize that some people learn to canter without first sitting it all the way: Riding the canter in half seat
When I started using a more forward seat and cantering in my *******ized half-seat in a western saddle, Trooper started moving out a LOT more willingly. Gone were the pinned ears. And when I rode like that, he would cheerfully canter 4 times as many laps with my 180
lbs as with my daughter-in-law's 100
I like the western style of riding, but one of the points that Littauer makes in http://www.amazon.com/gp/offer-listing/0668057912/ref=dp_olp_used?ie=UTF8&condition=used is that a traditional seat, whether in dressage or in its western variation, requires more effort from the horse than a forward seat does. The weight is carried further back, so the back has to work harder to move properly.
Based on a whopping sample size of 3 horses, none of whom are ever ridden in true collection, using a forward seat is a good way to get them using their back. When our weight is off their back, they KNOW how to move. They do it all the time, when we aren't on their back. So minimizing the way our weight hinders their back from moving allows them to move more naturally. With practice and as they get stronger, they can learn to move that way WITH our weight.
3 - Posting. Once your weight is off the saddle, posting higher only means you have further to come down. Maybe it is my own lack of skill, but I find the higher I post, the more likely I am do come down hard on the horse's back and negate the value of posting. So my goal in posting has become to post without getting a visible gap between me and the saddle.
It is also very important for me to get in balance with the horse. If my weight is behind the horse's motion, then I end up putting more pressure on the back. I do this intentionally with my mare sometimes. When Mia gets a bit rowdy trotting on a trail, I'll kick my feet forward and shift my weight back. It tends to have the same effect on her as encountering a section of extra rocky trail - she slows down.
As I said, I cannot imagine riding 50 miles nonstop, so feel free to ignore what I wrote. However, the book I linked to is an excellent book on riding forward. Used copies aren't too expensive, and it might give you food for thought. Good luck in your riding!
Oh - and foot position. As a ROT, shortening my stirrups seems to pull my heel further back. But a forward seat is different from a traditional seat, so I recommend reading a good book about it and/or getting advice from the jumpers. And I wouldn't worry about the head position. When the back can move, the head will follow. The head position needs to be the RESULT of the back, not a means for TRAINING the back. I can say from personal experience with my mare Mia that she could canter with her head inches from the ground AND a stiff hollow back!