The muse is a fickle mistress. Waiting around for her will only lead to a broken heart. The only way to get ink on paper is by jumping that muse and tying her to your desk. Sure, she will chew on her arm and possibly also yours, but it will be worth it. I mean, the only way to get anything done, is by simply doing it.
Since motivation can be tricky, the key is habit. Writing has to become as easy to do as brushing your teeth. As a child that may have been a chore but we stuck to it and now we don’t even think about it. The wonderful thing about this, is that it doesn’t matter how long the writing lasts. Just like with most things, the more you do it, the better you get at it. Writing is a muscle that needs to be exercised. Exercising for ten minutes is still exercising.
Finding Forrester was on TV today (one of my faves) and it was cool to watch a seasoned author type away on his typewriter within a second of sitting down. His mentee asking him what he was doing, wondering how he shook some amazing prose out of his sleeve within seconds. Because HOW is that even possible? I guess the same as when someone finishes a marathon. With a lot of practice. Whether you try to publish your work or not, the more you write, the better.
So take a notebook with you, or have your laptop within in reach and make sure you have a few moments to jot down whatever your fingers let escape. Your characters will appreciate it. And so will your (future) readers.