I am certainly an advocate of dreaming big. If, however, you don’t create a plan the feels feasible to you, there is less likelihood that you will follow through. I have experienced this myself.
I have a great idea, yet it somehow the end result or the way there doesn’t really “fit” how I can see myself. At first I am enthusiastic, but as the plan unfolds I begin to fall back into patterns that feel more comfortable. For me, I have learned that I will accomplish more over time (and maintain it) if I stretch myself within the realm of what seems possible today… stepping out of my comfort zone rather than into the Twilight Zone.