Us alcoholics have a bizarre kind of 'perfectionism' that we can allow to ruin pretty much anything. It goes something like this... "If it ain't perfect, f**k it."
I can just imagine that to-do list. I'm suspecting it was a perfectionists list. Like what the perfect student would get done. And then, because of a couple of things not being ticked off, you're getting ready to write everything off.
Cutting your run short sounds sensible - no runner needs injuries. You can build back up in the week when you're feeling less sore.
Okay - so maybe you didn't understand the assignment. When I think back to how I felt at 3 days sober, I can honestly say that there were a lot of things I struggled with, which I would usually sail through. The head-fog will pass. You will be fine. They wouldn't have given you a place on the course if you couldn't do it. Have you spoken to anyone about it? Another student or a tutor? (I'm guessing not, because we're pretty rubbish at asking for help or admitting when we're struggling).
I'm not sure why you'd think that you'd master the anxiety thing in 3 days. Give yourself a break. It sounds like you're putting yourself through a lot of pain by projecting and trying to deal with the future. Just try to focus on now. The chores you need to do now. Staying sober now. And maybe accept that in your first sober week you're not going to be as productive as usual. This will pass though.