It was habit that made me reach down with my left hand. Habit that told me I would find the bin there. I did it without thinking, as I do many tasks from brushing my teeth onwards. Writing is like a habit too. The more we write the easier it gets. I don't mean that the words are any better, but it becomes easier to get them down on the page.
The first story I ever wrote outside school was 400 words long and oh, how I struggled over it. Now, 400 words is nothing. I've got the writing habit, and it's easy for me to slip into the writing headspace. I write daily because I write daily. And because I write daily, it's easier for me to write daily. When (not if) I'm interrupted, it's easier for me to get back into my writing that it ever used to be.
Not all of my daily writing is novel writing. It's journalism and blogging and email and stuff like that. But the habit is there and it's all become easier. The more you write, the more you write. It's as simple as that.