Lets be charitable, shall we? They'd get pretty inventive, faced with 10 miles of walking taking a day, or on a bike taking an hour. Perhaps make hard rubber tires? Ones woven out of wire? Wood?

Bikes remain useful, even if they aren't up to modern road-bike standards. So useful, I think we'll see them for decades with ever more creative solutions to the problem of Bike Shop Closed, Oh What Will I Do?