Gordon B. Hinckley, who was the fifteenth President of the Church of Jesus Christ of Latter-day Saints among many other things, said "You can't plow a field simply by turning it over in your mind."
As a writer, you spend an awful lot of time in your own head, and that is a work of sorts, searching for plot and character along the winding paths of your imagination. But it's awfully easy to get lost in there, caught up in the planning, dreaming of where you could go. Ultimately and practically, once you've laid the groundwork in your head, a single written sentence is worth more than all the hours you spend dreaming up the mountains your going to scale. Naturally, this applies to life outside of writing, too. Take an entire day to think about what you could do and how you could do it and then take ten minutes to actually do something and see which accomplishes more. Mr. Hinckley knew that, even with prayer on your side, eventually, you just have to get off your keister and get to work.