Over and over, the runners lost their footing on steep limestone bluffs that spread out like tendrils on the western edge of the Texas Hill Country. The trails, if you could call them that, often amounted to little more than lengths of orange caution tape strung along the hillside. Loose scree—limestone rock deposited here more than 100 million years ago and then weathered and broken up into pieces just the size to trip a runner—demoralized scores of seasoned athletes while seeming to invigorate others. Many fell down, some repeatedly. The wooded hills and narrow valleys, which in the summer are home to an adventure camp, on this freezing January day were the setting for the Hoka Bandera Endurance Trail Race 100K, often described as the…