I was in a similar situation once, long hours of office work combined with a slowly expanding waistline and low physical exertion led to a deterioration in my health. I did what some recommended on this site, I bought Starting Strength (by Mark Rippetoe), joined a gym, and stopped eating like a pig and drinking soda. Now I'm slimmer, much stronger, no more back pain, and I enjoy going to the gym far more than I like playing video games anymore. Also I don't run, it hurts my joints (weightlifting does not), I use a rowing machine and long dog walks for my cardio.