You sit. That's it. There is almost nothing you can do about it. It's called work. Anyway, it's NOT dangerous.
Get a coffee, go to the men's room, get lunch, stand for a while, go to the silent reading room... That's automatic and stops you from being seated more than an hour at a stretch.
Add 30 minutes of walking every day (7 minutes to the office, 7 minutes home, 7 minutes to get lunch, 7 minutes to get back and then maybe to and from a meeting) and you should be OK as long as you don't stuff yourself with cheeseburgers and fries everyday.
If you on top of that add 4 hours of weight lifting (actually 1 hour lifting and 3 hours resting) you can get extremely fit; Gladiator contender or Fear factor kind of fit. Like I am.
Also, don't forget intermittent fasting everyday 16:8-style.