MINGSHUI—China’s e-commerce titans, facing a slowdown in the growth of their core urban customers, are battling to crack a new frontier: the sprawling countryside.

Each morning at 7:30 a.m., Pang Weidong starts his delivery route at an Alibaba Group Holding Ltd.-affiliated distribution center here in China’s northeastern plains. His small truck navigates pothole-ridden roads to deliver goods such as toilet paper and weightlifting sets. The 35-year-old Mr. Pang, whose daily route spans a length equivalent of Interstate 95...