In a similar situation and I understand where you're coming from. Although I have not taken on a Marketing co-founder, I have had a number of important and revealing conversations that I've learned a lot from.
1. Make sure the market for your product exists before you make it. This is counterintuitive to builders like myself and probably you. building is what I'm good at, and I know what I'm making adds value, but that doesn't mean others will see that value.
2. Kinda goes with #1, but talk to as many people as possible in the market you are considering a product for and learn what their problems and pain points are so you can solve them. If you can relieve someone's pain, the marketing will do itself. It's hard to even get the conversations, but each one is helpful, even if it's telling you something you don't want to hear.
3. Focus on gaining external interest, not necessarily revenue. Again, hard, but a pilot or partnership with another startup or small business can go along way and showing value to future investors or customers.
4. Talk to and learn from other founders. Your already doing this, but there are nuggets of information that can help even if it's just a slight change of perspective.
5. Keep your head up. It's a slog and it's brutal, but it's a marathon.
I'm still working on all these things myself, so you're not alone. The realization that a good product without a market is nothing, was a big revelation for me (and in hindsight makes me feel like an idiot). I honestly thought if you solve a hard enough problem or built a good enough product that the rest would fall in line, but that was not the case, but I'm not ready to give up yet.