It should perhaps be stated first of all that I'm not the most physically fit individual you will ever meet. I'm really not huge by any means, but I'm frankly I'm happy with myself. With my career path being what it is, it means that I will spend the majority of my time in front of a computer and there are some would use this as an excuse to never do anything other than be at a computer. However, the truth of the matter is that every often you really do need to get up. I'm not saying "Go run a mile" or "Start working on those six pack abs" I mean you can if you want I certainly won't discourage you, but at very least you need to just get up and take a little walk. Skipping over the physical health benefits: stretching your legs, helping prevent cramps, reduce eye strain, etc, it actually does a lot for you mentally.
Especially when you're stuck on a problem, just staring blankly at the screen isn't as helpful as you will probably convince yourself it is. We all do this, and usually it's a symptom of stubbornness more than anything else but when we encounter a problem we want to just keep plugging away at it until it's solved. Truth be told though, it's often much more efficient to actually walk away from the problem for a moment and then come back too it. Moving starts getting more blood and oxygen flowing to your brain simply by virtue of your body requiring both those things when you start moving. So get up, go for a walk, jump up and down, or just stretch a bit as long as it gets you to your feet. To be perfectly clear I'm not advocating simply goofing off, always be aware of your deadlines and how much you actually need to do but usually just getting up and moving around a bit will help more than you will ever imagine.