There isn't a quick/easy way to do what you're asking, but it is do-able.
First, you'll need to select the anchor points on the far ends of the expanded path. Use the Scissors tool to cut those anchor points to break the path into separate segments. Use the Direct Selection tool to delete the small paths at both ends. You should end up with two open parallel paths.
The next step is the "fun" part. Use the Blend tool to create a blend between the two parallel paths. The goal is to create a proper center line path between the two parallel lines. Go into the Blend tool's options to make it create a blend with 1 single step. Usually the blend will initially create a weird result between the two source paths. If you reverse the path direction of one of the two source paths it will usually solve the problem and make the center line fall in place properly.
If the original expanded path is complex in nature (has multiple bends, corners, etc) it will have to be broken into smaller segments with each segment getting its own separate single step blend. The centerline path would get assembled kind of like joining pieces of model railroad track together. Once the centerline is created and joined into a single path the line stroke width can be adjusted to a desired thickness.
I attached an example I threw together pretty quickly. I drew a curvy path with the pen tool, gave it a fat stroke and then expanded it. I used the steps described to create a centerline path between the two original parallel paths. I had to divide the artwork into several separate segments for the blend tool to yield a proper result. I have the paths colored to show the different segments.
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