Offsetting elements at a equal distance using along a curved element can be quite challenging with copy parallel tool.
For the most part using the move/copy parallel tools does just the tool implies, it will copy elements at designated value paralled to the orignal. The challenging part is when the element is not a straight line.
Go to the Home>Manipulate ribbon. Click on the construct array tool
Identify the path element by selecting the endpoint of the element i.e. arc,b-spline curve, or curve
(Along Path array type) Sets the way that the array is created.
See the following video for difference between move/copy parallel and construct array along path: