How would you advise one get formally started in a Product Marketing career track? I've previously worked in product management, sales and marketing and have just realized I've been performing many of the responsibilities of a Product Marketer for the last couple of years under a different title.

Well, it looks like you have many of the skills necessary to be an effective product marketer. Your challenge will be to tell a compelling story to a PMM leader that you have built all those transferable skills necessary to be a successful PMM. I'd expect your recruiting cycle may take a bit longer than someone already within the PMM track, and networking with multiple PMM leaders will be key to your success.