How can I achieve a domed or "hangar" type cross-section shape? Warning: I'm a day-two Blender user.

Specifically, I want the long, straight edges on the top face of the mesh to sort of "merge down" with the same edges on the bottom face (essentially, reduce it to one edge instead of two), with the curved ends ending up with flat bottoms and topsides curved in an even arc.

This shape was imported as an SVG, extruded and then converted to a mesh, but the geometry of it is super janky and overcomplicated-- WAY too many triangles and vertices. Ideally, the top face would simply be a series of long thin faces that line up with the sides. I tried to simplify the mesh in a couple of ways, but it didn't pan out and now I'm stuck. Should I just be reconstructing this shape some other way, or is this shape I've imported salvageable?



EDIT: I've been asked to clarify the end shape I'm talking about.
Shift+A Select Add a Mesh then select Add a Circle from the drop down menu. You can change the number of vertices in the tool bar on the left.

Press Tab to go into Edit Mode. Select the number of vertices desired to delete bye selecting them and then pressing X and then vertices from the drop down menu.

Select the bottom two vertices and press F to connect them.

Select all of the vertices and press E and then press the letter of the axis you want to extrude them along. For example press E and then Y to extrude along the Y axis.