How much training would a person need to start a half marathon?

There's a half marathon coming up in 3 weeks. I'll be honest, I haven't prepped much at all, if any. I was just doing my regular gym workouts with hiit cardio days once a week, and I run 1km before and after each workout session, jog for warm-up and a bit intense for the latter. I play sports once a week. And that's pretty much it, my cardio isn't all that bad but nothing close to what I need for this run since I just got to know about this half marathon.

I've been planning on doing one for a long time but I never got the chance due to illnesses and other hiccups. I'll complete this one even if I have to crawl to the finish line but preferably I'd like to prepare as much as I can in this short time span. Can anyone give me pointers? For diet I'm guessing high carb and protein for carb loading will be required, but when should I start that? How should I train starting today? And what else will be required? Also, if this is a seriously bad idea, like not just bad, but delusional BAD, I'll just shift to a 10k.