There's physical momentum and there's also psychological momentum.
Habits are a form of psychological momentum; there's so much psychological momentum that a habit is hard to stop.
When we need to get something done, it helps to have momentum on our side. It can keep us moving at a much faster pace than if we have no momentum.
What does this mean in terms of a dissertation or other large writing project?
It means that if you stop and don't work on it for a while, it's hard to get going again.
And it means that if you work on the project consistently and regularly, then it will become easier to make progress.
One thing it means is that you might do well to start slowly: put in the effort, but don't berate yourself for not making a lot of progress. The progress will come one you've built momentum.
To that end, it's often useful to set smaller goals, ones that you can finish easily just to get started.
For example, I left this blog untouched for months, and now it's hard to get back to it. And, truly, part of the reason I was able to leave it for months was because I had built only weak momentum anyway.
But now I'm trying to build some momentum anew. This is a start.