Ten years ago, when I was out of shape and started running, I discovered a secret. A secret that ultimately allowed me to complete eight marathons, improve my speed to the point where I was almost able to qualify for Boston (pretty good for a natural turtle), and somehow fit all the training in around my busy life.
I set small milestones along the path, and reveled in passing them.
With running, it was literal -- "I'm going to keep going until I get to that corner", and similar mental games to keep me going when I really wanted to stop.
I've used the same technique with writing. People often ask me -- don't you have a hard time putting together a 100,000 word manuscript. They question: my daily motivation, knowing where to take the story, how to develop the characters, and all kinds of other similar things.
The answer is -- not really. I develop a plan, and execute that plan all the while setting little goals along the way. And I try to mix things up enough to have some variety, just like I'd mix up my various running workouts (long run, recovery, hills, intervals, etc). If I just finished editing a manuscript, next I'll do a first draft, followed by the plot and character design for a new project, and then another editing pass on the the first manuscript, etcetera.
At the daily work level, I just plod along. I might try to edit two chapters, write 3,000 words, or finish one of the steps in the Snowflake process (my preferred design tool) in a day. If life gets in the way, I just say "Oh well", and take up the target the next day. But more often than not, I hit my target -- why? Because hitting those small milestones feels good, and I know it pushes me down the path to getting the big projects done.
Like today -- I finished the second draft of DELIVERABLES, my third novel. I started on January 7th, and it took precisely 24 days to read the entire document, and then edit 31 chapters. When I reflect back on it, I think it was completed pretty quickly, but I didn't feel rushed at all.