Schedule it on your calendar/planner. Put in an appointment to go to the gym or just get up and do foundation movements. Schedule the time and treat it like an appointment with your dentist or doctor. I schedule my workout time and other breaks in my planner. And when someone wants that time, they only get it if they are very good clients. I wouldn’t skip a cleaning for my teeth for a session or a visit to my doctor. Why is your fitness any less important?
Some of the newer fitness devices have a buzzer that will go off at set intervals if you’ve been sedentary for an hour, or 30 minutes, or however long you set it for. Then when it buzzes (vibrates, dings), you could get up, stretch, walk a few hundred steps to get good blood flow, drink a few ounces of water, then return to what you were doing.
That’s an easy way to get in extra steps no matter what your “base” level of activity is during the day.