You Can do this but you need to convert your instructions into geographic coordinates. For example purposes, let’s assume you have these coordinates (you would normally obtain these by geocoding your milepost or known points)

Start - Iowa border at US-61 (e.g., LatLng(42.0, -91.0))

North Point - 16.3 miles north (e.g., LatLng(42.15, -91.0))

Turn Point - Intersection with WI-133 (e.g., LatLng(42.15, -91.02))

End of Outbound - Potosi (e.g., LatLng(42.16, -91.02))

Return Trip - You can reverse these coordinates or define them as needed.

Insted of the instructions you need to send pints from where to where the google map build the map for you according to those points is this help