ON Monday, inside a warehouse turned gymnasium on an empty street in Gowanus, Brooklyn, a group of well-educated, accomplished New Yorkers chose to start their holiday with one of the most punishing workouts on earth. That workout, bluntly known as Murph, is as simple to explain as it was difficult to complete: a one-mile run followed by 100 pull-ups, 200 push-ups and 300 squats, followed by another mile — all for time. No picnic, and yet by 10:30 a.m., the gym was swarming with social workers and food bloggers, as well as F.B.I. agents and former Marines, who grunted and...