Be You.
I've been at my current job for three months. It's a small startup. I went from "where is the loo" to onboarding new employees in that time. Why? Because I'm an expert in my field. The landscape changes, but your expertise are going to carry you through. I'm having to re-learn a number of things I haven't had to think about in almost 20 years, but it's all there. Give yourself time to ramp up. And, one more thing:
Ask questions. Ask until you understand. DON'T BE AFRAID TO SOUND DUMB.
I tell new coworkers and managers this: I'm going to ask questions that may sound dumb- not because I'm dumb, but because I need to be able to relate information at its banging-rocks-together level. If they don't appreciate that, then I'm in the wrong place.