BALTIMORE—A half-mile into the Preakness, Mage’s Triple Crown chances were already in serious jeopardy. The Kentucky Derby-winning colt was racing comfortably—and wasn’t that far behind—but the pace was a fairly easy one, and that made the leaders tough to catch. “Not much speed in the race,” said Javier Castellano, Mage’s jockey. “The way it unfolded,…