This is why I think it is critical for a founder to do customer support for as long as possible. It should also be the last thing that is outsourced (or better: never outsource customer service!).
Customer feedback is the most valuable resource for a starting company. Understand what brings value to your customer, and understand what the pain points are. Where do they get stuck trying to use your product or service?
Apply what you learn from customer feedback. This is also a feedback loop on the improvement cycle of your product, as you should observe a drop in customer service requests once an improvement has been applied.
With every improvement cycle, the product should bring more value and less friction to the customer. This will make marketing so much easier as you now understand the customer's pain points, and you have a product that solves just that. Do it right, and the product will basically sell itself (you still need marketing though).