I ride three day 100's in the sand (Florida and NJ). I respect that sand
mightily and do not have the option in CTR of saving my horse by
dismounting. Since my mare was Grand Champion of the 1996 Florida 100, I
guess I did my homework well.
Horses will adapt their way of going to the sand, but if they've not trained
for it, you can end up with sore muscles, particularly along the rib cage
and in the groin (abductors?). The secret, I've found, is not to go fast in
deep sand.
There is an adage that you can condition for the mountains in the sand, but
not the other way round.
How much training? Just go by the signals the horse gives -- specific areas
of sweat, raised pulse, etc. etc. I would concentrate on sand training more
if I'm going to a sand ride than if I'm not. Trotting through mud and
powder snow can be good training, but be careful ANY time you change the
type of training!!
An oldtimer called the sand of the Florida 100 "Ball bearing" sand -- that
will give you an excellent idea of its properties!
BAM