What caused this pain, and how do I combat it?

HI.. I don't really have a history of back pain, but last week I began an electronics project that had me standing at my workbench for hours.. mostly with my head tilted down looking at the bench. Within 2 hours I developed an upper back pain between my shoulder blades. It got so bad that I had to retreat a few times to a lumbar-supported office chair. On the advice of my wife, I decided to order a posture support back brace. It came this morning and after using it all day today, the pain has lessened substantially.

I don't want to rely on this. I'm a 64 yr old male and still in reasonable shape. Is this a sign of weak back muscles? If so, what can I do to improve them?