You don't have to be super creative in order to illustrate for advertisers
I swear I'm not taking a dig at advertising (in this specific post), I just want to assure you that you most likely will not have to reinvent the wheel.
So take a load off!
You just have to draw well, quickly. Developing the skills to be a patient and effective communicator goes a long way. Do you need some level of creativity? Yes. I think it helps to be a strong creative problem solver. But if you're imagining the kind of creativity it takes to redefine a style, or make truly unique pieces of art, you won't need that most of the time. While some clients rely on your unique perspective to enrich their undeveloped concepts, most are looking for a technician who can reliably achieve a well established look.
As you prepare your portfolio: study perspective, time yourself when you're drawing, ask for help identifying your problem areas. Don't worry about being creative or developing your unique style when you start out in storyboards or sketch art. As your skills are built, your memory bank will fill up with solutions to future problems and your hands will inevitably show you how they want to draw. I strongly believe that if you want to survive in advertising, you only really need to nail three of the four: creative, reliable, proficient, relatable.