It's something you hear a lot when you're just starting out in endurance - any horse can do a 25-mile ride. Theoretically, they can do it several months after being Pasture Puffs.
The conclusion I am coming to is that, in fact, not every horse can or should do a 25. Maybe every Arab could do a 25. But not every horse.
I'm working hard to keep my admittedly forwards Haflinger in sufficient shape to do a 25 and not quit on me partway through. Her walk? Terrible. Her trot? Decent, and on flat terrain we could probably comfortably sustain 7mph or so on conditioning rides when I'm asking for it. I know we got a more forwards trot at the actual ride, so I'm comfortable saying she probably COULD do a bit more. Get near 10mph, and I'm riding a very extended trot, and not one powered terribly well from behind. She just doesn't trot that fast.
We're not conditioning on flat ground much, though. We have a mile/mile and a half of flat trail, but it's also heavily populated by hikers, dogs, and bikes. Then we have some lovely sections of hills. Same with real rides out here - 25s aren't generally on flat terrain that I'm aware of. So the pony and I end up booking it on the flat sections, recovering on the downhills (though trotting some in our LDs out of necessity), and power-trotting up the hills as her recoveries allow.
The other thing generally not mentioned, and perhaps that doesn't occur to most folks with Arabs or riding distance already - 5mph isn't slow. It's a heck of a lot faster than your average trail rider goes. My rides with friends? 3.5mph or so. That's not a sustainable pace for a LD. My 6-7mph trot is Too Fast for them. Your average trail rider doesn't necessarily want a horse with that much go.
On the flip side, I do have a friend with a horse whose trot is closer to 8-9mph without her pushing him at all. I think he could probably complete a LD just fine in a few months. I know they're out there. But at my little barn, there just aren't that many of them with that much forwards momentum.
Related: I made some point on the AERC facebook earlier this week somewhat to that effect. I wouldn't buy a Haflinger for endurance. Would I buy a Haflinger as a lovely all-around horse and do endurance if it seemed suited? Sure. I love my pony and enjoy riding her, 'slow' finishes that we may have, and we'll do LDs as long as she's sound and happily completing them. But looking specifically for my next endurance prospect? Probably not a Haflinger.