CASABLANCA, Chile—There are compelling reasons for dismounting from your horse and running alongside it in the fourth of five loops in a 120-kilometer (80-mile) race through dusty hill country not far from the capital, Santiago. Especially on a mid-December day at the height of the austral summer, when heat waves are hovering above the ground. And especially when you're going downhill.

"The penultimate loop," says veteran rider and horse trainer Pedro Pablo Gomez, "is what makes Copa Chile [the Chilean Cup] a very difficult race. The horses have already run 80 kilometers, the fourth loop is the hilliest, and it's crucial we reserve energy for our last loop."

I'm in the Veramonte Vineyard, in the dramatic Casablanca Valley, the staging ground