Currently, you can publish pretty much what you want, how you want, but I can see Amazon being forced to change that at some point in the future because they are being swamped by crap eBooks.
Why do you want to publish a chapter at a time? Is it because you are desperate to get your story out there and think it would take too long to write the whole book? That's not a great reason.
You have already written your story (so far) on your blog and on CycleChat so you can Copy & Paste most of it.
I'd suggest doing it as two books. Book one is the story 'so far'. It would describe how you got to where you were, how that made you feel, why you finally decided to change, how you went about it, and follow your highs and lows as you got on your bike and sorted yourself out.
Fat to Fit (that title has already been used!), and finally hitting your target weight. It's an inspiring story, but isn't the
whole story. The rest is for book two.
Book two needs to wait. Why? Because your story is currently unfinished business. I suggest that you set yourself the target of writing it after you have had your surgery to remove your loose skin and after you crack your SED and can finally eat a varied, healthy diet. In fact, when you finally feel relaxed enough about food to come on a forum ride to a cafe somewhere nice and sit down with some of your fellow CycleChatters and eat with us. When you can do that, your ride report would make the perfect final chapter. I'll take a photograph of you looking slim, tanned, fit and happy and biting into a tuna salad sandwich, surrounded by CycleChatters!
PS I can think of a perfect epilogue for book two - your run carrying the Olympic flame, should it come to pass! That would mean writing book two at about this time next year so that gives you a deadline to aim for.
