It's actually not nearly as dramatic as it feels. If your grades have been okay so far, you don't need to cram non-stop for months. I spent a lot of time preparing myself, but honestly the intensity mattered more than just the sheer number of hours. Realistically, I'd say you should start preparing seriously from next fall onwards, so like four to five months before the exam. That's not a crazy amount of time, but if you stick with it regularly, it's completely enough.
The trick is not to panic in January, but instead maybe start in September/October going over the topics that are harder for you for half an hour a week. That kind of workload builds up gradually - at some point you might be doing two, three hours a week, depending on how close you get to the Abi. What's more important is that you actually understand what you're doing instead of just solving problems. Old Abi exam papers from previous years are super valuable - that's how you get a feel for what the exam actually looks like.
In the last two months I'd really ramp things up then - that could be two, three hours a week. But it's not daily, more like focused study blocks. And super important: take breaks and don't slip into exam stress. If your fundamentals are solid, which it sounds like they are, you'll totally manage it. The nervousness is completely normal, but that's not a sign you're not ready.