How to write a book: Don't think about it (too much)

It's very easy to freak yourself out when attempting to write a book. (In fact, this is probably the easiest part, writers having such fecund imaginations.) Scarily, it can happen within a matter of minutes: you go inward, mentally fretting, and start turning your anxiety settings up. It's too big a job. I can't do this. I'm not a writer. What will I do about the sagging middle bit? Suddenly, The Book has grown into an insurmountable, fruitless goal. The result? You procrastinate, draw the process out over painful decades, or even give up on your book completely. 
'It sounds like crossing an ocean,' an old friend once said after listening to me whine about the enormity of the task I had set myself. A yacht skipper, he collected brand-new boats in Cape Town and sailed them across seas to their owners in the Mediterranean and Caribbean, sometimes spending weeks without seeing another boat in the distance.
'You can't think about that whole ocean ahead of you,' he said, shaking his head. 'If you do that, you'll never grasp how you'll actual make it across. You'll think it's impossible. The journey seems too long, the sea too big, your boat too slow. I mean, a yacht is just this piece of stuck-together wood with a few ropes and stuff... How on earth will you and your crew of two make it to the other side of the world?'
I asked, So what do you do?
'You play a mental trick on yourself,' he revealed, taking a sip of his beer and smiling like a Zen master. 'You settle back, relax, and just focus on what you have to do that day: say, travel a short stretch, from this marker to that next one. You make lunch. Have a sleep. Steer the boat. Keep watch. Chat to the others. You tick off your task for the day. Repeat every day, and after a few months you're amazed to see that you've actually arrived at a foreign shore. Your tiny daily achievements have added up to something major.'
Similarly, as a novelist it helps to stop focusing on the end point. Instead, ask yourself, 'What small piece of my manuscript can I work on today?' Maybe relook that section of dialogue that doesn't feel quite right, or tinker with your opening paragraph, or write the beginning of that key scene you've been picturing in your mind for a few weeks. Action beats inaction hands down. Do something, anything, and you'll probably feel freer and lighter. Plus, with each passing day, you'll have come closer to a new destination.