How do you create an inline in AI?
The problem is a logical one. There is never a unique solution for an inline, and a lot of guesswork has to be done, especially where strokes meet (e.g. in the middle of the M). Therefore it cannot be automated.
What you can do, however is get the middle of two (compatible) lines through interpolation:
- On either end of the stroke, select the two end points and Opt-Delete them. This should break the path into two paths.
- Double click one of the two paths, so it gets selected entirely.
- Right-click and select Reverse Selected Contours from the context menu.
- Choose Paths > Selection to Background (Cmd-J).
- Press the Delete key to remove the selected path.
Filter > Transformations > Background with 50%, confirm.
Now you have the middle line. Of course, this only works if both halves of the path are actually point-compatible, otherwise step 6 does not work.