I was once an athlete—tennis player, basketball forward, and, God help me, a cheerleader, with a kick that could slice granite. And I’ve been quite content to coast on my former glory. I’ve long taken for granted some metabolic good luck and scaffolding that, dressed, makes me look fitter than I am. So I hardly noticed that with each passing year, my once-muscular frame was morphing into a chopstick. Until my son, a ridiculously fit quarter-miler, challenged me to a single push-up.