Biggest marketing challenge?

One of the biggest challenges I've found recently is to truly know if you're selling / serving your real target audience. Or if you're missing out by selling/serving to the people you 'think' are your real target audience. Especially in the early days! Currently learning this from building my own platform, and reading The four steps to the epiphany by Steve Blank, who talks about the importance of customer discovery (finding your customers), and Customer development (adapting your product to really know if you're building what customers want/need).