Dark urine as an indication of muscle damage?? Tom, please reply!!

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Kristi iluvdez@yahoo.com
I'd really appreciate knowledgable opinions on this.....
2 days ago we did an easy 3 hour conditioning ride. Right
after, because of a very large brain fart on a human's part,
both horses got loose and immediately challenged each other
to a race. They took off down the road at a dead run. To make
a LONG story short, it was determined that they covered about
10 miles in 40 minutes before they were caught (miracously
they avoided barb wire and road founder). One horse seemed no
worse for wear metabolically, but mine didn't fare as well. 
Although his HR, resps, and cap refill improved quickly, he 
refused to eat or drink until we arrived home (an hour later).
As soon as we got home, he urinated and it was a very dark red.
Because his attitude improved almost immediately after that, he
never moved stiffly, he ate and drank well when home, and his
2nd urine an hour or so later was medium golden, I didn't seek
vet attention. (Maybe should have with the dark urine??). My vet
inspected both horses 24 hours after their "race", and her advice
was that my horse should NOT be exercised (other than pasture
turnout) for at least one week, and preferably two, because of the dark 
urine that I saw. Her 
explanation was that the dark red urine contained large amounts
of hemoglobin, which signified severe "muscle shredding". Tom
(or anyone else), what can you tell me about what happened to my
horse? Was it the equivalent to a human marathon runner "hitting
the wall"? What's happening in his system as he recovers?  What can
I do to optimize his recovery? I of course feel just sick that this
happened to him, although I am SO thankful he wasn't seriously injured.
Advice and ideas from experienced endurance folk are very much
appreciated!! Kristi