My children will also ask, "How do you eat an elephant?" and then tell me, "One bite at a time."
I've talked before about the value of incremental effort for writers. If it takes a hundred thousand words to make a book, or a hundred hours to make an edit, or a hundred books read to make a good writer, then you're going to have to get used to taking small steps toward your overall goals. The important thing is to guide your steps in the right direction. And keep taking them.
Michael talked about having a routine to boost writing productivity, but I'd like to take that a bit further into mapping out your goals such that you have specific, reachable goals you can hit on a daily basis, a weekly basis, a monthly basis.
Back in 2013, I met my agent for lunch and griped that I'd been stalled at 35,000 words on my novel for far too long. I forget what I said, but I effectively gave myself a kick in the pants. Lent had just started, so I turned it into Personal Novel-Writing Lent. A thousand words a day, no excuses. I feel my writing is a God-given vocation, and therefore for me it fit nicely into the category of "spiritual discipline."
I got a notebook and my fountain pen, and they accompanied me everywhere. I had to estimate what that thousand words was since I was hand-writing it, but 40 days of Lent should net me 40,000 words (with the occasional day off for taking a sick child to the ER.) That would put me in striking range of the ending.
Nice and measurable. (I screwed up the measurements and ended up with more than 1k per day, but that's fine.) Sustainable.
After that, the next goal was to have the entire hand-written portion of the book entered into the computer in two weeks. Definable.
Then a nice round goal of one month to do my first- and second-pass edits. Get it off to beta readers. Incorporate their suggestions. (That part was tricky because it wasn't solely dependent on me, and you can't rope other people into your goals.) And then have it to my agent by June. Then get it on submission by September.
I outlined every step and knew what it would take to achieve it, and having that goal every day kept me on target. You can't eat an elephant in one bite; similarly, a book-length manuscript is huge. You can't just spit it out.
The incremental effort link talks about taking 34,000 stitches to knit a pair of socks, but one of the ways you stay on track with socks is having a pattern, and having an idea of how much time you can commit to knitting. Or the inverse: if you know it takes 20 hours to knit socks, then you know how much time you have to give the project if it's September 23rd versus if it's December 23rd. (Er…just trust me you don't want to be doing that.)
Books don't have spiecific patterns, but the same way you can say, "I need to reach the heel turn today," you can also say "I need to reach 45,000 words by the end of today," or "I need to get the first draft of my query letter written by 9pm" and eventually "I need to decide by Thursday which of these agent offers to accept."
Having a goal you can hit means you will be more motivated to try. Higher motivation boosts productivity. Good luck!