Writing a book doesn't have to be a difficult task. Yes, it can be time-consuming, but if it is something you really enjoy - writing a book as well as illustrating it, can be extremely rewarding - especially if you know, you have done the lot.
I think the hardest part, has to be slotting in regular periods during the day or evening to partake in the act of writing. It is really easy to get side tracked and say to yourself, 'I'm not really in the mood to write,' or 'I'm stumped - so I won't write today'. Both are not really useful to developing consistent opportunities to write. A little everyday gets you into good habits; just like brushing your teeth and yes, you'll get so expert at it, your work will begin to shine. You just have to make time, no matter how small.
150 words written everyday, over the course of a month will give you a 4,500 words start on a YA novel - usually around 60,000 words. This means you'd just about be finished writing your book, in 1 year. If you put in double the effort each day - you'd be finished in 6 months - double that again and you'd be done in 3 months!
Remember, the 10,000 hour rule will only make you accomplished, three times that will bring you to virtuoso standard. :)