On Monday through Friday, I nanny a 15-month-old baby who calls me “Shoes.” I feed her, read her a book, then throw her in socks and velcro sneakers to dump her into a stroller. We walk for miles, go on swings, and say hi to my cats through the screened window facing the street.

As Shoes, my life has been nonstop. Before Baby’s lunch ends on Tuesdays and Thursdays, I slip on my sneakers and run, hefty laptop-laden purse on my shoulder, to catch the Broad Street SEPTA. I sit, shining with sweat, a...