KML (or KMZ) only can represent a line as a collection of points with at a minimum a start and end point.
https://developers.google.com/kml/documentation/kmlreference#linestring
You could calculate an end point from a starting point, angle of direction (or heading), and distance then represent display it in Google Earth using KML.
For example in the OpenSextant geodesy java library you can create a Geodetic2DArc and calculate the endpoint in 3 lines of java code like this:
Geodetic2DPoint start = new Geodetic2DPoint(new Longitude(-49.3456789, Angle.DEGREES),
new Latitude(-12.3456789, Angle.DEGREES));
Geodetic2DArc arc = new Geodetic2DArc(start, 5000.0, new Angle(123, Angle.DEGREES));
Geodetic2DPoint endPt = arc.getPoint2();
The distance is in meters so if you want a long line segment then you need a larger distance.
Then with the associated Giscore library you could export the line into KML directly with few more lines of java code:
KmlOutputStream kos = new KmlOutputStream(new FileOutputStream("out.kml"));
Feature f = new Feature();
f.setName("line");
List<Point> pts = new ArrayList<Point>(2);
pts.add(new Point(start));
pts.add(new Point(endPt));
f.setGeometry(new Line(pts));
kos.write(f);
kos.close();