In Lionel Shriver’s entertaining send-up of today’s cult of exercise—which not only encourages better health, but now like all religions also seems to promise meaning, social superiority, and eternal life—an aging husband’s sudden obsession with extreme sport makes him unbearable.After an ignominious early retirement, Remington announces to his wife Serenata that he’s decided to run a marathon. This from a sedentary man in his sixties who’s never done a lick of exercise in hi