Sore back

I recently have had a sore back for a month. This is because I rarely exercise every day. When I was in Japan, for my commute, I waked from my house to a station, stood in a train and walked from the station to my office. When I think back, this were really good for keeping up my health. Now, living in California, I drive from my apartment to my office and walk from a parking lot to my office (just 3 minutes). How on earth can I keep my health in this situation? I should go to a gym in my apartment and exercise as soon as possible…