I know people with full-time jobs, small children, and busy social lives who still find time to write. Frankly, I don't know how they do it, although at one time so did I. Now I'm retired, so theoretically I have time to write. You know what they say, "In theory, there is no difference between theory and practice, but in practice, there is."
In my head (the theory part) I will write or edit or polish so many pages a day. I try to write between 8:00 and 11:00 am, then take a walk, have lunch, and relax until 1:00 or so, then work again until 4:30 or 5:00. I do this Mon-Fri and give myself the weekend off. No pressure, and I can stop to throw in a load of laundry or run to the post office.
But those stops often become gaps and even whole chunks of work time that slip away (that's the practice part). My husband needs help on a project or wants to go for a ride to pick up some things. Or I get sidetracked when I notice that, ohmygosh the flowerbeds look terrible. Sometimes something as simple as a blank crossword puzzle will derail my work plans. I'm a little compulsive, and those squares just beg to be filled in.
I think a writer needs both the theoretical schedule and the freedom to break away from it. After all, we aren't manufacturing car parts here. We're building worlds, and sometimes those worlds need ideation (great word probably invented by ad men). Part of the creation process is making yourself sit with pen and paper or keyboard or whatever and pound out a story, and I know dozens of people who have been planning for years to get to that point but haven't done it. But once you have started writing, another part of the process is letting your brain play with that story. The walk, the chores, even the crossword puzzle may contribute detail or unknot a plot point. I'm guessing that every writer has to find her own balance point. If you only get an hour before the kids get up in the morning, then you'd better make the most of that hour with actual writing and let your brain work on the ideating in the background as you go about your daily tasks. If you can actually plan five or seven or nine hours of writing a day, then you'll need breaks so you can step back from the story and see if it's going where you want it to. (Your back will thank you, too.)
So I say the making a schedule part (again, theory) is very important. But keeping it in practice requires a lot of adaptability. You can do it. Your brain will help.