Good luck!
As you say, 6 months is a long time to keep motivated. If it was me I'd declare this to be the start of half marathon training, and then consider after the Sydney half to be the full marathon training - which still gives you about 16 weeks. It's never too early to start on the mind games...
Very exciting on the spreadsheet front that you're counting the training in fortnightly blocks rather than weekly.
200km per months sounds like a sensible target. And as we all know, there's gold (medals) in them thar hills.
Keep us posted!