Do you have a personal computer? Or access to one at the library? Type out what you want to say in a word processing program and save it. Then if its your own computer, highlight the enitre document and click "copy". Switch to the internet and open a "post" or "reply" window like you would if you were typing in a comment. but put your curser in and click "paste".
Following my last comments, I thought of a couple of other things.
Doing this, you will build a library of your posts. I like that and in fact if you do ever write that book (I'd buy it in a heartbeat) you'll have the rough draft.
Also, you'll have access to spell check.