What was your worst mistake as a first-time founder?

I didn't appreciate talking to customers as a way to validate my product.

I'm seeing so many people here with products who obviously didn't spend even an hour talking to customers (else they'd have a much different landing page, a much different product so to say.)

As @csallen said, start with your solution LAST. Why? Because it's the most FLEXIBLE thing you can change in all sorts of ways. You can't change the problems your potential customers have. You can't change the acquisition channels, nor the rules they have. Thus, why some people (like Brian Balfour) came up with principles like a product-channel fit.

Focusing on "building it".