When I was working (read: employed) I used to write on my days off. The people around me knew that Tuesday was writing day. After I retired I gradually became less disciplined. There was more time -- too much, perhaps. I became easily distracted. If I was writing and I thought of something that needed attention, I'd go look after it. It could be as trivial as adding an item to the weekly shopping list, or checking to see if I should water my houseplants.
Finally, I realized that if I wanted to be more productive, I needed a regular routine. Besides, the muse has to have a way to reach me, and with my mind running all over the place, she wasn't having much luck.
I made the decision that I must be ready to go to work every morning at 9. I must have had breakfast and be dressed for the day. No more writing in my PJs. I keep a pad beside me as I write, and if something off-topic comes to mind, I write it down. It can be looked after when my work day is done.
Now, I have to confess that I had let email become 'way too important to my day. I had to make a new rule for myself: NO CHECKING EMAIL UNTIL YOU ARE READY TO BE DISTRACTED. Reading email early in the day, which is when I happen to be at my creative best, has been totally distruptive for me. If I didn't reply to it immediately, I'd be thinking about my responses. Or I might read an email that unsettled me, got my mind going in another direction entirely. That's the kiss of death for me!
It's not a hard schedule to follow. I take a short break at 10:30, the same as one does at a regular job. Then I go back to work till 12 noon. I have lunch, check my email and go for my walk. The afternoon is when I look after any household chores and prepare the evening meal. But I find that after an uninterrupted morning of writing, my mind is still so absorbed with the story that I return to the manuscript frequently during the rest of the day. And this is good. Because much of the preparation for writing takes place in my head.
A writer needs self-discipline, and this is the routine that works for me. I'd love to hear how you schedule your writing day.