Having a broad plan like you have done is great and an essential model to making your idea a reality. What you have described is a common process to creating a startup whilst still employed.
What you should now do is start building in smaller steps in to each phase of your plan. That will help validate whether your timeframe is correct. For example in your first phase (late November 2011 - March 2012, 4 months) you might have an idea of what is required to get your software to beta: whether it be a minimum set of features, a staged development model, or a target number of beta testers to attract.
The further you refine and break down your timeline the easier it will be to stay on track. If you knew that at the end of each week what needs to be achieved you will far more likely stay on track. You need to track your progress and keep a steady pace... even during those downtimes when you start asking yourself why am I doing this again?
Like the saying goes... by the mile it's a trial, by the yard it's hard, but by the inch it's a cinch. Cheesy, but effective.
As you progress you will hit roadbumps, opportunities, all manner of distractions that may affect your schedule. Keep measuring where you are, where you need to be, and work out the steps to get there.
Just don't lose sight of your goal! Every action you take should be inching towards what you want to achieve.