I've found that serials operate according to a slightly different set of rules than regular novels. I wrote fantasy gaming stories at EnWorld (here's an
example) for about eight years. The trick (at least for me) was to have a lot of cliffhangers and characters that readers could develop relationships with. In fantasy there are a lot of immediately-recognizable archetypes that make this easier to do. You don't necessarily have to have your characters in a life-or-death situation at every break point but it helps to have a reason for your readers to come back for the next installment. And your first post has to have something in it that snags the reader; I suppose it's not that different from the first few pages of a novel in that respect, but given the post length you cited in essence you are selling the entire story in a very compact introduction. I tried to make my posts about 1.5-2k words and that worked well for me, but then again my stories were much longer (the one I linked above ended up being 633k words).
Good luck! Serials can be a lot of fun.