The superfit walk among us. They saunter or strut, depending on whether they’re showcasing their magnificent agility or their oxlike strength. They ignore the chatter in the health media over treadmill technique and pedometer steps. They scoff even at seemingly rigorous practices like Mysore Ashtanga yoga and marathon training. They are America’s self-styled fitness elite, adherents of a punishing online exercise regime called CrossFit, which orders its followers to cultivate a distinctly martial — not to say paranoid — ideal of “physical preparedness.”