She founded a $400 million fitness app and became a self-made millionaire at 22—but Kayla Itsines says real cash flow came after buying a gas station

A millennial entrepreneur has reeled in millions from her success as a fitness influencer, but now she’s making bank and paying rent through another unlikely source: a gas station. 

Australian fitness mogul Kayla Itsines has built an exercise empire over the past decade, amassing 15.6 million Instagram users tuning into her content. 

The influencer first tasted success with her 12-week “Bikini Body Guide” (BBG) fitness program—a business model she cofounded and bootstrapped...