Exercise is good for you. I'm not talking about abandoning your office chair or going for the burn, although taking a physical break regularly is undoubtedly wise. The kind of exercise I'm thinking of, is for your writing 'muscles'.
It can be anything from doing a newspaper crossword or wordsearch while enjoying a cuppa, to writing a paragraph about what you can see through your window. There's no need to be stuck for ideas. As long as you have a dictionary, all you need to do is open it at any page and write something involving the first word your gaze falls on. Could you write a paragraph without prepositions or articles? How about avoiding adjectives or subordinate clauses?
You could choose a colour, or one of the senses, and brainstorm everything you can think of connected with them. Take 'blue', for instance. You could write something about its symbolism, blue eyes, a blues club, feeling depressed etc. Maybe you could combine a colour with a sense.
Why not write a description of someone you have seen? It could be someone who passed you in the street, someone in the news, or an acquaintance. Write about the same person in different genres such as a news report, romance or memoir. Try combining things that you would never normally put together–a man in a pin-striped suit on a farm, perhaps.
What you write can be off-beat, full of purple metaphors, or only use words of one syllable. it doesn't matter whether what you produce is 'good' or nonsense. What matters is that you'll be stretching your writing ability and giving your brain a work-out.