Probably a bit against the grain, but I don't think you need to try and act like you are an experienced PM. No amount of blog posts or books will quickly get you to that level, only experience will. They were well aware of your background when they hired you. Perhaps they hired you _because_ of it? At a company that size, PMs are often just corporate animals playing politics a good chunk of their time. You'll probably have to become more similar to them over time, but for now, you might just have a honeymoon period where you can add your own flavour to how the product you're assigned to should be run, and make it more successful.

As a founder, you probably already have a lot of the skill set that's needed for that. If you listen to people and apply your intuition, I bet you'll do well.

Sure, understand what the role is generally about, what the expectations are and all that. But I don't think it's a problem that you didn't hold it before, no need to make it one. PMs are in my experience a slightly different job at each company anyway. The most important thing with your background is probably to develop an eye and tactics for the games other PMs and middle managers play.