Duplicate your initial shape [ctrl+d]. Use the pen tool [b] to extend the ends of the shape and close it. Duplicate the new shape [ctrl+d] then [Path > Path effects...] [Offset]. Set the offset distance to a negative value. Convert the path effect to a path with [Path > Object to Path]. Use the node tool [n] to remove unwanted segments. Reduce the number of nodes with [Path > Simplify].
Let’s say I drew the Black Line.
Now I want to draw the Red Line.
As you see the distance between the Black Line and the Red Line is 3 cm.
How can I draw the Red Line?
Duplicate your initial shape [ctrl+d]. Use the pen tool [b] to extend the ends of the shape and close it. Duplicate the new shape [ctrl+d] then [Path > Path effects...] [Offset]. Set the offset distance to a negative value. Convert the path effect to a path with [Path > Object to Path]. Use the node tool [n] to remove unwanted segments. Reduce the number of nodes with [Path > Simplify].