Once you get used to writing every day, you will simply do it. It will be rarely necessary to muscle your way through it. It will be easy to sit down and write. I've always argued that beginning the writing session is the only really hard part. Once you are there, you will get something done.
I think it was Aristotle who pointed out that habit is a stronger force than character. In other words, what defines us is what we do habitually. To be someone who reads a lot of novels means to read novels many days out of the year, not to be someone who, in theory, is a reader, but can rarely be found in the company of books.
Today I was thinking that I was not getting much done by adding only 80 words to the chapter before I go to the gym. But by exposing myself to what I had already written, I was also allowing my project to get its work done on me. I have enough written on this new book (16,000 words of notes with a little bit of finished prose) that just by reading what I have written already I can inspire more of my own thoughts.