Writing a novel shouldn't be draining on your brain - though it will be a mixture of hard work, determination and dedication (time & effort). To help you on your way I have found that following these steps has helped me develop my writing;
Map out your ideas, no matter how far-fetched or daft they seem, note them down. They can always be developed or deleted at a later date.
Don't keep re-drafting your first draft as you go, get it written first and then go back to it to redraft.
Be prepared for the long-haul. Writing a novel takes a lot of time and effort and especially not giving up.
Research, research and even more research! The more you do, the more knowledge you accrue and can add depth to your novel. Though, you don't need to write all of your new-found knowledge in your book - this will make it boring and more of a non-fiction novel. However, if you want to write something that entwines the two together then Jonas Jonnasson (The Hundred Year Old Man Who Climbed Out the Window and Disappeared, The Girl Who Saved the King of Sweden) is an awesome writer who manages to achieve just that.
Know who your target audience is. There is no point writing a scene about mythological creatures if your target group is crime or romance. You also need to be aware of the age of your audience - young children would not be reading books with swear words in, nor would it be appropriate to be graphically violent.
Create a writing space where you feel comfortable and at peace. This can be the library (brilliant for research and reading other books), the beach (calm and soothing), nook at home (comfortable and familiar), the local park or anywhere else that works for you.