Seconded. I used to feel lower back pain due to the amount of sitting at a desk. Running and lifting weights did wonders for how I feel on a daily basis.
That also makes things like using a standing desk, or taking breaks to walk around, a lot easier to do. That weight hanging out in front makes everything but sitting painful.
Exercise didn't help me with my occasional back issues, but switching to a standing desk really did. I didn't get one of those adjustable ones, because $400 for a desk that raises and lowers was silly--instead I just got a draft-height chair and stand and sit as appropriate.
At some point, I decided I wasn't even going to bother with a chair. Every time I sit on the tall chair at work, I'm just Not Productive compared to standing.
Just running and lifting before or after work doesn't necessarily solve the problem though. I was talking to my doctor about it, and she said it was equally if not more important to take regular breaks during work to walk around, "look at something far away" (computer job), etc.