Whatever you think of middle-aged cyclists, I envy them their exhilarated faces. Rarely do I see joggers who inspire me. Middle-aged joggers, with their glazed eyes and their slackened, panting mouths, give me the impression they’re trying to hold back an infarction before breakfast.

Lately, I’ve been wearing the same expression. By the time I jog around the park and head for home, the twinge in my knee has become a roar. My lips are set in a grim line. I lead with my left leg to take the pressure off the right. My face is drippy with sweat and florid with exertion.