Do you have a date for your meeting yet? Did he say it had to be done by then, or was it someone from Target?
I did a version of AST before this one, but I think they are essentially the same. It was self paced, and I had as much time as I needed--per my DM. I think you can pick up what you need in an 8 hour shift. Lots of common sense and things you learned already during your barista training. I would suggest that you read through it, and take notes if you are unsure about anything. I was never questioned about AST tbh. Did you do it? Yup. Ok, good. I don't really remember everything that was in the AST training, and have never referred back to it.
There are a couple of different types of DM visits. There is a DSV, Detailed Store Visit, which is about 3 hours long and these happen quarterly. It will start with a coffee tasting. You will discuss how the store is doing. Bring sales figures, Dashboard, and a completed SOA. For your first visit, he may ask about your training in general, and AST in particular, or he may not. He may ask about staffing, and may want to see your team's barista certs, so have them available. He will probably walk the store, checking on how clean things are, if there is anything missing, dates, and the general condition of the store. He will probably observe the barista working for a while. Then, you will sit down and make an action plan for the coming quarter, so it is good to have an idea of what you want to work on.
There is also the Observe and Coach visit. It is also scheduled, and is about an hour. It is more informal. I always start this one with a coffee tasting too, but we both like coffee, so. . . . We go over sales and upcoming promotions. He will walk the store, and observe the baristas. It sounds pretty similar, but less in depth, I guess.
Schedule yourself as an extra person for the first visit, not as coverage. I always do this. You will still have to be back up, of course. My DMs have always jumped behind the bar if we get backed up on a visit.
You should remember that your DM is there to help. He can be a resource for you. He will point out what needs fixed, because its his job, but it really is to make your store better.
What exactly is stressing you out, or is it just all of the new stuff?