The job of a PMM can vary greatly from company to company, industry to industry, and even based on where they report to (Product vs Marketing). What have you found to be successful in defining your career as a PMM?

Yes, in my experience there are definitely a few org considerations that will shape your fortunes as a PMM, and navigating those will determine whether you're set up for success. 

1. The first, as you allude to is the function you report into. I've been in orgs where PMM has been in marketing, product or sales. IMO, PMM should always be part of marketing because, while you're no doubt deeply embedded with the latter two functions, you are by definition a marketer and need to be evaluated as such. The problem with being in product or sales orgs is that your contributions--while appreciated--will not be top of mind when it comes to calibration and performance assessment. And that's just physics--a PM leader will likely dole out comp and stock refreshes to the top PM talent they want to retain. Ditto with sales, which can be an even harder case to make as performance is quota based and you are the "cost center". Note that it's fine and even a good idea to be dotted to product/sales. 
2. The second is more company specific, and has to do with the dominant company culture. You've probably heard of companies that are more sales centric, or others where engineering is "king". Now you can do amazing work as a marketer at either one of these, and grow your skillset commensurately, but do pay attention to how "valued" marketing really is as that's what will ultimately affect your long term growth trajectory. Things to watch out for: PMM viewed as a "service bureau" i..e "so we're launching this product in 3 weeks and need a blog post" or if you feel stuck constantly doing reactive and ad hoc work vs fleshing out programs and shaping strategy. 

As you can tell there's a bunch of situational stuff and pattern matching you should try and stock of as quickly as you can. But if you're at least confident that you're set up for success from a reporting and cultural standpoint, then the rest is up to you. And that's the fun part.  

You're absolutley right! The first starts by understanding how much emphasis the company puts on distribution vs. product. An observation I've made is that companies who have a CEO with a sales/marketing background give PMMs more authority over product development and GTM. Companies with CEOs with an engineering/product background tend to give PMMs a more limited scope focused on market validation, positioning, and messaging. 

What has made me successful in a PMM career was starting on a nascent product in which we were "firefighting" every single day. I thrive in startups or ambiguous environment when we're trying to prioritize what's important. 

When I first joined, our programs in my product group were not complex to begin with. It was about getting the bare necessities out to customers and sales in order to drive demand, mature pipe, and close deals. I shuffled from function to function, experimenting and quickly learning about all the levers I could pull to drive revenue. 

I always think of PMM as the glue that connects the various functions of a business (product/sales/rest of marketing/etc). A PMM's responsibility is to understand the needs of prospects and customers and communicate that internally. Additinoally, it's PMMs job to take all the innovation and programs a company is creating and wrap that in an easy to consume message/narrative for prospects/customers/employees/analysts. 

Hoever, if you're in product marketing and find yourself reporting to Product, that could be a challenging situation. I believe Product and Product Marketng should be peers who work together.

1. Ensure that your focus areas are aligned with what is most important to the company. For example, if Enterprise is the core segment, you might not get the support and interest you need/want if you are focused on B2C. Doing what is most important to the company means that you are delivering value where it matters and where people have investment and attention paid.

2. Ensure that you are doing what you are most passionate about. I always say that we manage against two objectives in any job: doing what's best for the company but also doing what's best for you. So in the example above, maybe B2C is just your passion. You've tried Enterprise (which I always recommend anyways) and you still want to grow your career in B2C. That's fine - perhaps then it means finding another group or company that is aligned on that value, and grow from there. People typically give more in areas that they are passionate about.