If all haul roads should start with an m-value of 0 and end at whatever the distance is of the route and you have individual features for each haul road within the attribute table, it seems like you can add a From_Field and To_Field, calculate a 0 for the From_Field and the length of the line for the To_Field, (you can use the Calculate Geometry function), and then create new routes using the Two_Field method. This will autopopulate all of your routes with the m-values with respect to the digitized direction of the roads. You can use this method for new routes only and possibly merge them with the old routes.