Shooting flash with the 5D II

My wife let me use her 5D Mark II and her 580 EXII flash at our 60th high school reunion. I left it in P mode and took shots in a poorly lit hall.

It gave OK exposures but the shutter remained open for about 1/2 second after the flash. It seemed that the shutter was making an exposure based on the available light and disregarded the flash exposure.

What I got was properly exposed and sharp foregroud where my subjects were located and lite by the flash and a properly exposed background that was lite by available light but blurry to camera movement during the 1/2 second time exposure.
Tell me what I did wrong.