I have worked in a startup before (in fact, I have worked only for startups in my life) but I don't think that's necessarily a prerequisite to starting your own

I have worked in a startup before (in fact, I have worked only for startups in my life) but I don't think that's necessarily a prerequisite to starting your own. When you enter a "classic", funded startup as an employee then you won't get in touch with the meaty details outside your area of expertise more than you would in a conventional company. Thus the "new" aspects about founding a company remain new, even if you went through one as an employee before.