Convert a solid filled shape into a single line.

What about converting to raster then using image trace outline?

I WISH there was a simpler way to convert a fill into a stroke. Im a motion designer and EVERY graphic asset and template I download are compound paths that can't be used because they bascially did a  "Object > Path > Outline stroke". It's easy to turn a stroke into a closed fill. But not the other way around. Adobe, if you're reading this can you add a feature where this can be? Like "Object > Path > Create Strokes from Fill" or something?

Sincerely,
A Tired Motion Designer

Step 1 - Create Object without stroke

Step 2 - Select your solid filled shape & newly created object

Step 3 - Using the shape builder tool deselect the unwanted object and delete the solid filled shape.

Step 4 - Convert a remaining object into stroke.

Monika is correct — just use Offset Path if you want to make the shape thinner. Incidentally, if you really want to turn this particular shape into a single path, it should take you about 30 seconds to make it from scratch. Take a circle, delete one of the four arcs, rotate 45°, flip and duplicate, connect two of the endpoints, and extend the others a bit.

James,

To convert all the shapes to lines is almost impossible, as you know all the shapes have X and Y dimension. For example how can you convert an ellipse to line?
Ok, maybe, a line with a good profile can do that. But what about the star? The result is smaller shape.

If you have one or two shapes, you can use all above mention methods. But what about some lines of text?

There is another approach that can convert shapes to lines, but it works best with letters and numbers.

First you Rasterize  (object>Rasterize), and then you Image trace, using LINE ART preset.

I hope this helps!

To make it thinner just use Object > path offset path or Effect > Path > Offset path with a negative value.

Antistandard,

You may (save a copy of the original path so you can fall back on it):

1) With the Direct Selection Tool Click each end path segment and press Delete (that will give you two paths forming the curved parts);

2) Select everything and switch from Fill to Stroke in the Toolbox (that will give you stroked paths (and you can see what you are doing));

3) Object>Blend>Blend Options, set Spacing to Specified Step with 1 step (that will prepare the right blend);

4) Select everything and Object>Blend>Make, then Object>Expand Appearance, then delete both the original paths (that will give you the stroked path midway between the curved paths), then set the Stroke Weight as desired..