It'll be late April - as 2012 is Sunday the 21st, its likely to be Sunday the 20th in 2013.
The ballot entry system will be just after the 2012 event, and you'll have roughly one day to enter before it closes. The ballot gives you a 1-in-5(ish) chance of getting in. Otherwise its a charity place, with associated fund-raising commitments.
There are other marathons, so I'd recommend you follow your plan of trying 10k and then a couple of half-marathons and then decide if you want to try the full marathon. You may then prefer to pick a marathon that doesn't have the same rigmarole to get a guaranteed entry.