Well... I'm not a mental health professional. So it wouldn't be appropriate for me to suggest whether-or-not what you're experiencing rises to the level of Imposter Syndrome. To me, as a lay-person, what you describe simply sounds like new-job jitters. Starting a new job is always (at least I think it's always) anxiety producing. Perhaps it is more-so or less-so depending on the personality of the person involved. If a person has a tendency to be anxious anyway, then of course it's likely starting a new job may be more anxiety producing than it would be for someone who is overflowing with self-confidence. (That can sometimes create problems of its own!)

Based on what you wrote, I would say just give yourself time to adjust to your new job, ask questions and request assistance as necessary (avoid overdoing this though.) Perhaps talk with a family member or trusted friend outside of work about how you're feeling. Talking it out can help. (Also get plenty of exercise to help burn off that anxiety.) And if talking with family members and / or trusted friends isn't an option for you then consider seeing a counselor or mental health therapist for a while to help to allay your fears.

It sounds to me, from what you wrote, that you have the skills to do this job. You're simply experiencing those new-job jitters I mentioned above. There is, at least to my mind, something of a danger here in blowing all of this out of proportion ruminating over the possibility that you have some serious mental health condition called "Imposter Syndrome". If you make the new-job jitters you're experiencing seem serious enough you may just turn the whole situation into one of those self-fulfilling prophesies. My best wishes to you for great success on your new job!