Is this something I can convert to miles or meters? What is the equation?
If M value is a measure value, there's no information about what unit it might be using when the xy coordinate system is geographic. The spatial reference doesn't have a unit for the measures.
If the xy coordinate system is projected, you can guess that the measures are using the same unit, but that may not always be the case. As an example, the measures might represent mile markers but the xy coordinate system might be using feet.