I've read several articles recently stating there is no reason to pull up a horse who is laying quietly. In my experience if a horse is laying down, that's fine. IF they can lie quietly until a vet is out that's ok. If they are rolling that's very bad. Also if they are up, down, up, down, up then keep them up. Laying down because of a belly ache isn't the bad guy of the situation, its rolling - The reason for walking a horse is to keep his mind off from rolling, not laying.
If you are on a considerable amount of sand and worried about the potential of sand colic to a test. You can see if and how much sand your horse is getting by putting a handful of poop in a jar or rubber glove. Fill with water and swoosh it around some then let it settle. If I remember correctly the sand falls to the bottom, not sure how much is to much. Its something you can call and ask a vet on.
Many years ago back when my horses lived on nearly a sand box my vet said to on occasion use Equine Enteric Colloid as a preventative. Never had an issue so it must work. Its a a natural colloid fiber specifically designed for horses to aid in expelling and eliminating foreign material such as sand and undigestible feeds from the digestive tract.