I ran two little pseudo-science experiments on my mare. I don't think it's really following the scientific method if you only have one experimental subject, so I can't announce this as the reproducible truth, but it's good enough for me.
I've been intermittently confining Dixie's mane on our rides, and I thought that it does make her sweatier to have that thick wad of hair on both sides of her neck. For Saturday's ride, I braided her mane on one side, and she really didn't get very sweaty at all, despite about 15 miles of intermittent trotting. So today I left her mane loose, flopped down on both sides of her neck, and rode 7.78 miles at an average of 4.3 mph. That's .1 mph faster than the NEDA ride, and less than half as far - and she was drenched when we were done. Sweat all over her neck and chest, on her back, even her flanks were sweaty. That's good enough for me - ponytailing/braiding her mane does make a big difference in her comfort, so I'll start doing it all the time.
Second, I thought that perhaps she was gaiting better when I wasn't sitting all the way back in the saddle. She racks ok on the flat, but she hit that probably-a-running-walk going up a hill when I was leaned forward in two-point. So for today's second experiment, I stayed well forward. I actually learned three things from that! One, it kills my shoulders to ride like that. Two, she possibly gaits more - I got some really smooth pacing and step-pacing. Three, it's bad for her back. She had dry spots at the front of the saddle area. Woops. I won't do that again. I poked at them and she didn't seem to care, so I don't think I did any permanent harm to her.
I took some more pictures of her hooves. And I got a decent shot of the weird toe rocker I was talking about:
Then I accidentally learned more about it! This post on the easycare blog talks about what I'd call a foxtrot, and why the author thinks it's a good thing for endurance horses. In the section Quarter and Toe Scoops, he says:
Similarly, the toe experiences increased abrasion during the break-over and the propulsive components of support phase. High-mileage barefoot (unbooted) horses often develop a toe-scoop on their front feet: a slight depression of the hoof wall and sole at the toe between (roughly) 10 and 2 o’clock. Like the quarters, this toe scoop simply reflects an area of high wear.
That's pretty much what Lytha said and what I suspected. She's just wearing her toes off in the sand/rocks.