Knowing what to do is 10% of the work. The remaining 90% is actually doing it. In a growing, understaffed startup, there are always fires to put out. Unless you can stay focused, you will never get the big, important things done. Here's how you can get it done:
Create a roadmap similar to what you’ll have for product development and work in sprints (typically two weeks). It will be easier to communicate to the rest of the organization what you’re working on, and it will help you stay focused.
Before each sprint, look at all the initiatives you have in front of you, prioritize based on expected outcome vs resources needed (time and money). Make a rough estimate of how much time each item will take. In the beginning of the sprint, sit down with the team and break down initiatives into smaller tasks. This will let you estimate the required time more accurately.
Use a project management tool like Trello or Asana (or a whiteboard and Post-its) to keep track of your tasks and progress.
Make your roadmap available to the rest of the organization. Allow them to have input on what the marketing team is working on. The best ideas could come from anywhere, and your work is dependent on others and vice versa. Internal communication and transparency will make everyone’s work much easier. Embrace feedback, it will make you better.