It's not unusual for backpackers in Muir Woods National Monument, north of San Francisco, to wander onto Green Gulch's 115-acre valley and book a room for the night. Located on the Pacific's boulder-strewn sand, the Gulch, as everyone calls it, sits at the nexus of dozens of trails and offers workshops in foraging – but don't let the idyllic setting fool you. "A lot of the Zen students here are runners, swimmers, and bikers," says Shokuchi Deirdre Carrigan, a Buddhist priest-in-training and coordinator at the center. "They are people who like to challenge themselves." Whether you're a banker who runs marathons or a doctor who does Tough Mudders, Green Gulch bets you like intensity in your meditation – and thus offers a seven-day sesshin: a silent, seated Japanese Zen style of intensive meditation that lasts from 5 am until 9 pm each day. "It's its own kind of marathon," says Carrigan. "Sitting still for a day is a physical and mental discipline like biking a long distance." [From $90 a night; sfzc.org/ggf]