Since you don't have a mobile device at the moment, I would focus on general OpenGL programming to start and get the basic concepts down, since they will be the same whether you are on PC or a mobile device.
If you go to http://www.reddit.com/r/opengl and look on the right side, there are some really good tutorials that can help you learn modern OpenGL programming.
Its definitely possible to learn in 6 months if you are a hard worker and not afraid to ask questions.
I've been using win8 since it was released, and while I wasn't sure about it when I started, I am happy with it for the most part. I'm coming from Vista though so the improvements in stability, speed, etc, all seem great. I'm getting used to the new right side menu and not having START and it works pretty well, I think. Main thing I don't like about MetroApps is that you can't use OpenGL with them.