Mostly what everyone else said. The skill barrier between Casual and Advanced is largely an illusion, and if you're used to doing three paragraph posts regularly you're already operating above the minimum standards of the section.
The one thing I would add is that Advanced section elitism is real, albeit less prevalent than people make it sound whenever they talk about it. If you get some twat telling you you're awful because of some grammar mistakes, or because you're not writing a novella for each post, or whatever, don't let it discourage you. The minimum for an Advanced post is two paragraphs and coherent writing ability (says so right in the subforum description), and anyone who says otherwise is a deluded idiot. A GM may ask for more in their game, and that's perfectly fine, but the elitists tend to hold the foolish notion that their arbitrary personal standards are the Advanced section standards that everyone needs to adhere too or else they're awful. Those types are a tiny minority though, so you might go a long time without encountering any, but it's better to be forewarned well in advance than to be blindsided.