On a rainy Saturday last summer, I can be found paddling out, popping up, and generally hanging ten—goofily, shakily, semi-triumphantly—in a skylit East Village walk-up. It is as awkward a fitness challenge as any I could imagine—“Everything just feels unnatural when you start surfing,” says the beneficent Aaron Thouvenin, cofounder of Surfset NYC—and one that...