Get up out of that chair; it could kill you. Well, not exactly, but people who sit for long periods of the time have a higher risk of obesity, high blood pressure, high cholesterol and heart disease than people who are not sedentary. Some studies have even linked long periods of sitting to an increased risk of premature death from a stroke or some forms of cancer. However, standing for long stretches, compared to the same amount of time sitting, doesn't make a huge difference in health outcomes.
As with many things, the answer is moderation. If your job requires you to sit at a desk for an entire day, the best way to combat some of the health risks is to get up every 20 to 30 minutes. Even just a quick walk around the office cube farm can keep muscles from going limp and improve blood flow. This combined with a healthy diet and a moderate amount of exercise per week (150 minutes is the current recommendation) can keep you healthier as you age. If you want to stay independent in your later years, the key is making the right choices in your younger years.