The great thing about being the first hire is something that is also great about Product Management: there is room for interpretation. My philosophy has always been more heavily focused on understanding how things operate current state, finding out pain points as well as the more successful parts of a product, and leaning on those insights to form your next steps.
In certain scenarios, what a team may need is for someone to roll up their sleeves and do the work to keep the lights on for a product. It may be months before you can get the product to a comfortable enough place to think about weeks, months, or quarters ahead; however, that time allows you to gain knowledge of the product itself.
In other scenarios, a product may be operating just fine, and your task will be to understand where it can go next. Your time will be spent with customers, stakeholders, engineering and others to understand the areas of opportunity that exist. You don't always need to reinvent the wheel or start from zero just because there was not a product team in place.