I already had a leg up because I had most of my product put together. I just needed to figure out how to market it.
And the first thing I learned was that my product was dead, would remain dead, and was a waste of further time and effort. I fundamentally had begun in the wrong place and needed to first find what people wanted.
You need people you can connect with, provide value to, extract value from - and you have to make all that into a reproducible and reasonably predictable process given all of the constraints in play.
If you can do that, you're often well down the path towards a viable business. If not ...
Research has backed this up over & over -- that story about Sony's focus groups about the yellow vs black Discmans is just one example -- not to mention anecdata from, well, everyone.
That's why I teach my students to observe customers and deduce, rather than ask them.
Watching five subjects in a row get to one point and then get confused is important, but they might never mention it if you just ask them directly afterwards.
I've also observed talk between users that started out "oh, I hated it" - then they'll go on to praise the functional aspects - and it turns out they just didn't like the color scheme. Which is equally valid if it generates consistent responses.