How do you recommend someone break into enterprise product marketing after only having start up experience?
Try for IC roles at giant enterprises like Amazon Web Services, Google, or Microsoft. These are large organizations and have several teams with new initiatives that almost operate like a startup within a giant org. Your startup skills will be very valued by these teams. And you will have end-to-end scope across strategy and tactics even as an IC - like a mini-CMO for your area. Once you get in and have some experience under your belt, you can find your way over time into other teams or other enterprises.
Always focus and leverage your strengths. What do you bring from the startup experience that can be valuable to the enterprise setting?
Some ideas:
The startup was agile and worked with more uncertainty and you're more resourceful - you're better at dealing with these scenarios/projects
You had to "play a lot of instruments" - you have gained experience in many different topics - Compared to more specialists in enterprise teams, you can bring an integrated view to the job.
Some projects that could fit:
A multifunctional team was set to explore a new business opportunity or segment. Needs someone who can think as a generalist and can handle uncertainty or an ill-defined problem.
The sales team is lacking support - you need to work with them and quickly understand what's happening and come up with ideas and solutions that can bring together several teams/skills - CS, Product, Content, DG,... - you need to identify, problem-solve and act in a short period and involving many different functions.