Marketing is a huge field.
You could easily divide it into numerous categories:
Advertising, public relations, community relations, promotions, social media strategies, website design, strategy, research, budgeting and revenue projection, etc.
You can't be good at all of it.
In fact to be great at some elements of marketing you may have to actively ignore other parts.
It's vital that you gear your arts marketing toward your strengths. It's the only way you can get the most out of your limited time and money.
The good news is that you get to choose what you want to be good at.
Maybe you decide that you are going to deliver quality visuals via your web and print advertising and not spend much time worrying about marketing research.
Maybe you decide that you are going to focus on community outreach and marketing research and not advertise much at all.
Each of those options could work.
What doesn't work is trying to do everything . . . and ending up being mediocre or bad at all of it.