Do not sit for prolonged periods - sitting leads to obesity. Put your laptop on the floor and stay on your tummy when you use it. If you are alone, eat while standing - put your plate on the counter in the kitchen. When you have company, you will obviously want to sit down together, but while alone you can manage without sitting down. Exercise does not help if you are too sedentary, especially in terms of longevity which is a bit early for you to be concerned about, but still. Sitting itself is really really bad so cutting out as much as possible is the way to go.