It can be daunting... especially with the big stories. Knowing that there's going to be hundreds upon hundreds of pages ahead of you can seem insurmountable (for me, it'll be over a thousand pages easily). But every chunk you write gets you closer to being done, whether it's the first draft, third draft or the final one. And really, the hardest part is getting that first draft down... trying to put all your ideas into a cohesive narrative. Once you got that, then it comes down more to cleaning it up, making sure there's consistency, adding more bits in to help flesh out what feels too short, or trim down what seems too drawn out.
There's something inspirational when you sit back and look at what you've done. When you see 100 to 300 pages behind you, it gives you that little "you can do it" nudge to keep going. Knowing that you've come that far, and that it's taking a real tangible form instead of being only a mental idea, is a good motivator.
And thanks Refelos