You never know what you are capable of until you try it.

But before you start, put down in writing things that will tell you, "I'm over my head, time to stop" and stick to that - if you are not handling it, bail.

Now you can go to the job knowing that they think you can do it, and knowing that you know how to protect yourself. If you succeed this time, then you have learned something good about yourself. If you aren't yet ready, you didn't lose much. And if you discover your own limits but they found you useful, you will be in a better position then to get them to agree to something less ambitious.

With that decision made, you can now take things one step at a time.

Cheers,
Ben