What really helped me - I started attending boxing training. At first, it was nothing, but fitness with(and without) gloves, a lot of running, a lot of jumping and so on. But then body got a pretty good shape and my long sitting at office became not so painful. I realized - it was what I need. I has been attended boxing training session twice a week for a last three years. And my back's muscle corset became so strong that I can sit a couple of hours without break and don't feel tired because of that. As a bonus I can knock out anyone who is standing on my way. ;-)
P.s.: please, do not forget to stretch your body after each training session - it's very important (to give your muscles some sort of relax command)!