Ask HN: How to approach first days on a new job as a senior PM?

Inspired by this post: https://news.ycombinator.com/item?id=42656184

I'm starting a new job in a few days as a senior PM at a ~1000 person company, but I've never been a PM before. My career path has been: PhD -> Engineer -> Founder.

My time as a founder has given me some unique perspective on products in my space, but I'm less experienced with the day-to-day of a PM in a medium sized company. My exposure has been second hand watching the PMs while I was an engineer. Any advice on how to help ensure things kick off well?