You can extend a drawing tool as a ray (extends to perpetuity in the direction you drew the line) or as a line (which extends into perpetuity in both directions). To do this, double click the line you drew to open the Edit Modal, you will see a box for “A ray?” and also one for “A line?” A ray will extend the line out in the direction that you drew it. A line will extend the line in both directions:
Once set up, you can also check the box to "Save as Default" and then all new horizontal lines you draw will automatically have these settings applied.