Fittit, trying to get back in shape, having some issues

I'm a recent college grad now out on my own. I don't have the spare cash to get a gym membership right now so all I have is a dumbell(up to 40lbs per) set and the Insanity videos. After the stress that was my senior year, I had put on about 30 lbs (Now 6'2, 220).

I started my old running regimen of 3-5 miles a day 4 days a week, but immediately started getting lower back pain. I was a computer science major so I spent the majority of last year in a computer chair programming, inevitably I developed poor posture. I've since begun wearing a light back brace to help keep it straight while I work, but the pain is still there.

I'm looking for some good exercises I can do to help strengthen my back muscles so I can make running feasible again. Not having done the Insanity stuff before I'm not sure if that will help or hurt. I'm no stranger to exercise, but structured workouts are fairly new to me.

Thanks for your help in advance!