I understand the self-doubt thing, but as others have said, if you were honest about who you are and won the job, then clearly the employer sees good in you and has confidence in your abilities. As you said, you know you would do everything you could to do a good job, so you are ethical and responsible. If you put aside your self-doubt and really would otherwise want to do this, then go for it! Once you are there and engaged, the fear and hesitation you have now will be a thing of the past and your confidence will blossom as you achieve goals put before you.

What a great opportunity!