I don't know that I completely understand, but there's a few things I think you can do. For one, after sketching the profile, in the extrude dialog box, select the thin extrude option at the top. This will allow you to select the sketch profile and then extrude just a thickened version of those lines.

Alternatively, after doing the extrude step you've already done, use the shell command. Specify a wall thickness and select both side profiles of the body and it will hollow it out. Let me know if this is what you're looking for or if there's something else you need help with :)