There was an article about this in one of Running Bear's running magazines a couple of years ago, and he managed to get me quite worried about it. That's when I installed Workrave, which I have set for a short break every twenty minutes. (The period the magazine recommended.) Interestingly, I then discovered that in the summer, I seldom get to sit that long anyway.
In the winter, Running Bear works primarily as an artist, and works from home. He's downstairs and I'm upstairs, and he brings me cups of herbal tea at frequent intervals (he's a tea jenny ), saving me the bother of going down to the kitchen. He's also nearer the door to deal with callers, etc.
In the summer, he works primarily at his cycle hire and repair business at the other end of the village. I then seem to spend half my life running up and down the stairs - to put the washing out, answer the door, make a cup of tea, answer the door, put more washing out, chase after the coalman to pay him, make more tea, realise it's raining - quick, rescue the washing - and so it goes on. (We get a lot more callers at the house in the summer, because lost tourists wanting bike hire keep being misdirected here by helpful locals. You'd think Google maps on his website would solve that problem, but no. )
We do try to go for a walk together most days, and in the winter we generally manage to fit in a longer walk, which (hopefully) makes up at least in part for my lack of stair-running.