Some jobs come with some major physical risks.
Writing is not one of them. Our major risks are back pain and carpal tunnel. It does not take huge amounts of physical strength and prowess to be a writer (one of the reasons I like it). That does not mean, however, that we should abandon concern for our bodies. I truly believe that our physical health directly effects our mental and emotional health. Just in that sense, it behooves us writers to take care of ourselves physically.
My biggest concern about investing time and energy in taking care of my body is the time it takes, which could be used for other things, like, for example, writing. But it really doesn't actually have to take that much time, and the time it does take is worth the sacrifice. Scheduling in time for your body will help everything else run more smoothly, I think.
There are lots of options here, in terms of exercise. Join a gym, recruit a workout buddy, everything from that up to training for marathons. There is one type of exercise that I'd like to highlight, though, one that I think is perfect for writers: taking walks. Simple and easy. You can put just as much time and energy into it as you want to, and best of all, as you're walking around you see new people and places, experience new things, smell new smells and hear new sounds, all of which are creatively inspiring. If you can find a convenient way to carry a notebook with you, you could even walk to your favorite cafe or park and do some writing.
So, the moral of the story is, buy one of these:
They'll keep you on your toes.
So happy walking and happy writing!