If you eat well and go to the gym for an intense (ie. not just jogging on the treadmill) 1-hour workout every day, I think you'll be in quite decent shape.
Actually, no. Sitting for large periods of time, past a certain age as I remember, is simply harmful to your health. You can't "make it up" with exercise later in the day.
You don't make it up the negative effect of sitting, but you'll be in generally better shape, perhaps by a greater margin than someone who never sits but also never gets any strenuous exercise.