Starting a new SWE job soon and I’m terrified

This was my first SWE job out of college. I’ve spent roughly 4 years working on legacy codebase bug fixes and in general being a slacker at a big corp company. I’m starting soon at a small start-up that has multiple greenfield projects going on, so I’m excited to finally get to write some code. But I’m also really fucking scared that I won’t be capable of performing at the level they’re expecting me to. I keep feeling like this company made a mistake in hiring me because I have no clue what I’m doing and feel like I’ll need lots of mentoring. Is there anything I can do to “prepare” for the job or make sure I hit the ground running once I start?