I make this assertion because I have a very nice Thoroughbred mare who 
won very nicely at 5 1/2 and 6 furlongs, placed at 6 1/2 furlongs (this 
is at Hollywood Park too, mind you) but just 'didn't have the distance 
in her' when they stepped her up to 7 furlongs.  Initially, I supposed 
that this was because she just wasn't built to go distance (she is a 
little tied in at the flank); however, having had the opportunity to put 
some LD conditioning into her, I suspect that the a bigger reason for 
her inability to go 7 furlongs had more to do with her not having been 
schooled to control all that speed.  When I first started riding her, 
indeed the only things she knew were 'break from the gate and go like 
hell.'  Over time I have been able to get her to respond to the aids, 
rather than standing up on her hind end when she couldn't have her way, 
and I honestly believe that she 'has a lot more distance in her' than 
she was given any credit for at the track.  She was wasting too much 
effort in disobedience.
We, as endurance riders, could learn a lot from the TB race people.  
Though I am not advocating that all endurance horses have a certificate 
from the gate steward certifying that they are 'safe in the gate' before 
being allowed to start, I think we could take a page from their book and 
understand that there are certain standards that we should all try to 
meet before bringing our horses to a competition, and that these 
standards include standards of behavior and levels of training, not just 
whether the horse is in good enough physical/metabolic condition. We 
could learn a lot from what they do, from what they don't do, and from 
what they don't do which they should do.  
In the same way that Thoroughbreds running 1 mile need to know more than 
break and run, so do endurance horses.  The safety issue becomes even 
more relevant at an endurance ride where, though there is less speed, 
there are many more horses, much more varied terrain, longer distances 
and hence more time for mishap, and most importantly...novices and 
children.
I believe, in England, that horses are required to have successfully 
completed some novice rides before they are allowed to compete in open 
endurance competition (some UK list members would know more about this, 
I could be wrong, but I think I remember reading about it once).  I am 
not suggesting that we should have rules requiring that all horses and 
riders have passed some kind of test (I rather like the AERC's general 
policy of 'anything goes'---I realize that I am over-simplifying here), 
but not because I don't think there are minimum standards of behavior we 
shouldn't all be able to meet, but rather because I am not a fan of 
rules, and think it should be the responsibility of each individual 
participant to make sure that his/her horse is sufficiently well 
conditioned AND trained to compete.
kat
Orange County, Calif.
p.s.  I have also had the opportunity to 'reschool' a lot of Quarter 
Horses off the track.  I have yet to hear of anybody trying to 'rate' a 
quarter horse in a race--those horses know virtually nothing.
p.p.s.  I would love to correspond with you privately about 
Thoroughbreds, as they are really my favorite kind of horse.