Here's a secret: Writing books never gets easier. I've written 23 of them, some best-sellers, some duds, and each one is as tough to complete writing as the previous one. You should take some comfort in this, as it won't get any harder than it is now.
If you have a book to get out of your system, then you have the examples of millions before who have succeeded. Here are some high-level suggestions.
Write in short bursts, then stop: Author Todd Henry limits his writing to 50 minutes a day - even though he makes a living as a writer! The best-selling Die Empty author finds that the solid, definitive limit makes his writing time more productive, helps him avoid burnout and trains his brain to focus, particularly if he writes at the same time every day.
Read your writing out loud: Best-selling food writer Michael Pollan swears this is the key to good writing. If you trip over a sentence or have a hard time saying something, then it probably means you need to edit more. Reading out loud also helps you prepare for any potential book tours or speaking engagements you may have in the future, not to mention if you want to do an audiobook.
Consider it a business card: To paraphrase best-selling author James Altucher, books are the new business card. Even a book that sells only five copies can be valuable to you if it represents your viewpoint, philosophy or skill to potential collaborators, customers or clients. In other words, don't put pressure on yourself to create a masterpiece. Instead, do your best to convey your perspective in the book. Great writing is worth nothing without a clear perspective - one that is uniquely yours.