This is so important to us deskbound writers.
You have probably heard of the dangers of deep vein thrombosis to airline passengers? How many of you realised it could also affect YOU or any crazy office workers putting in longer hours on their butt than they should be. You need to get up and walk around regularly at the least.
You should consider getting yourself a standing desk so you can move more. It's possible to make one using just books or something under your monitor, mouse and keyboard, or you can buy a standing desk, or you can create one like the setup in the picture, if you're a handy carpenter. I like to go from sitting to standing and vice versa.
Here's a link to a post about deep vein thrombosis and desk sitting.
Here's a post about some of the added health benefits like burning more calories, improved posture etc.
The longer you sit, the shorter you live. There's debate about this but it's common sense to me - we weren't made to sit on our butts all day. When you move your leg muscles you're helping that venous blood to move back to your heart instead of pooling in your legs.
USA Today - sitting death article.
Here's some great detail about why and how to get a standing desk.
And this post here, I found wonderful for figuring out the best heights for everything as well as a stack of other information.
Do yourself a favour and move more!