If you're just starting out in GTD don't focus on the higher horizons at first. You have to get control of the basic levels of work in your life in ascending order. First get the runway and 10,000 feet (Projects) under control and then you can move into higher levels of strategic thinking. Your "stuff" at runway and 10,000 feet will seriously impair your ability to think at those higher levels. Give yourself 4-6 weeks to get the runway and 10,000 under control first, then start defining your larger horizons.

Your mind needs time to trust your system. You'd be surprised how much the mind resists the unfamiliar even when it makes sense. I was such an out of control crazymaker basket case when I first started that I needed two months for my mind to trust the system enough to let go.

You'd also be surprised how much "stuff" is baked and caked in the deep recesses of your mind. Once my mind started to let go all of this "stuff" started to bubble up especially while I was driving. Get a voice recorder that you can operate without taking your eyes off the road -you'll need it. Turn all of that "stuff" into projects and someday/maybes and then you'll have bandwidth to think about your focus areas, goals and your mission in life.