Hit 1 on the number pad to go to front view.
Hit E to Extrude and then Z to stay on that plane
Hold in Ctrl and move the mouse up. See image.

Okay now were cooking. Before we go any further lets make a few small adjustments.
Hit A to select all vertices.
Hit Ctrl-N. Recalculate normals outside.
Look on your panel for Modifiers.
Click the add Modifier button.
Subsurf Catmull-Clark 2 levels. See image.
Depending on your memory constraints adjust the levels however you like. I suggest 2.

Now De-select all vertices toggle A.
Hit 7 to go to top view.
Hit (toggle) Z so we can't see through any faces. Solid
Hold in B on the keyboard until your curser becomes a circle with crosshairs in it
Start selecting those neglected outer vertices that are in the groove. See image.

Hit 1 on the number pad for front view.
Hit G to grab.
Hit Z. we want to stay lined up.
Hold in Ctrl and use your Down arrow key.
Keep hitting down until you see -0.2000 along global Z on the left of the panel.
Hit Enter.

(I merged the image together D: -0.2000 is on the lower left of the screen )