I think it is really important to treat your writing career as you would any other career, namely, I take it seriously and treat my job like an office job by sticking to a routine. (This applies when I'm working on a manuscript, which is most days right now. When I'm not, I have more flexibility!)
Generally, I get up, take my kids to the bus, then take a quick walk. I like to have gotten moving and given my brain some time to wander/free associate before I have to hit the computer screen. By, say, 8am-ish, I plop down and try to hit my word count for the day, somewhere between 1k and 2k. Because I've given myself time to prepare mentally for this, with the walk, this usually takes an hour or two, rarely more. (What I mean by that is that for most writers, the actual writing isn't what they spend the most time on ... it's the getting lost in their head and figuring out what they want to say that takes up time.) Then I'll squeeze in a workout and get back to my desk, answering emails, screwing around online, rewarding myself for the morning's work.
I take a lunch break to run errands just as I would if I were in an office.
The afternoon is spent either returning to the manuscript, if I'm so inspired (but because I ideally hit my word count, this isn't obligatory) or handling non-manuscript related items. For example, when I have book promo to do, I have a lot of Q/As to get to or essays to write, or if I have a celebrity interview to draft, that sort of thing.
And then my kids are home from school, and I become their Uber driver for their afternoon activities!
I am actually a night-person, so if inspiration hits, I may return to the computer after dinner.
But I rarely deviate from this schedule if I'm knee-deep on a project. It just streamlines my day and helps me stick to a writing routine, which I think is absolutely critical for anyone trying to complete a project.