This short exercise will introduce you to what some 3D modelers have called the most powerful tool available to modelers, the humble extrusion.
Select two vertices along the bottom side of the shape you made, as shown here on the left.
Press the E key, and use the mouse to create another face as shown on the right.
We discovered that if you press the E key when you have two vertices selected in Edit Mode, then Blender will create a new face. This is called an extrusion. The last two Time for action sections seem too simple perhaps. You built a face, and extruded another from it. But, this is often how modelers create objects, such as automobiles, which often have fluid lines and no definite edges for much of their surface. Making faces and extruding them, you can make just about anything.