This article describes the entrepreneurial development and professionalism of ultradistance running (ultrarunning) in South Korea, culminating with the hosting of the IAU World Cup 100K in 2006. This case study-based research provides evidence of various macro-environmental and individual drivers of a grassroots entrepreneurial process, contextualised in Korea’s sporting culture. Macro-environmental factors include the economic crisis and Korean cultural values of comradery, emotional expressiveness and entrepreneurial spirit. At the individual level, self-leadership, focus, persistence, team dynamics and access to resources explain the growth of ultrarunning and the commitment to hosting the IAU World Cup. Implications for sports entrepreneurship and grassroots-initiated mega-sporting events in Asia are discussed.