After dark, Brazos Bend State Park feels like a place that, if you’re not careful, could swallow you whole. On the chilly, rainy December evening when I arrived at the park (in Needville, about an hour southwest of Houston), thick fog obscured the few buildings, and the forest, framed by enormous live oaks whose long, meandering branches were draped in Spanish moss, formed a dark, menacing wall along Park Road. A pair of eyes, probably belonging to a raccoon or a ringtail, glinted in the darkness. I traveled here to meet runners participating in the Brazos Bend 100, an ultramarathon in its tenth year. Marathoners came from across Texas, 34 other states, and five countries for the event, a choose-your-own-adventure affair that also offers shorter distances,…