She’s fast but in no hurry. Mary Cain first made headlines at 15, with a promise so bright that famed runner Alberto Salazar was soon phoning in tips, breaking his rule against coaching teenagers. The last time an American girl dominated running was the ’70s; she broke multiple records but endured 20 operations blamed on relentless competition. Cain, now 18, is protected by coaches who took those lessons to heart. She could break the 1,200-meter world record — if slow and steady really does win the race.