One thing to really help you out, do yourself a huuuuuuuuuge favor and don't go detail crazy in pre alpha. Just simple shapes and plain textures. Says the guy who is famous for going absurd detail crazy in pre alpha, of course, but I'm an idiot so don't be like me.
The first thing you want to do is NOT open up Hammer right away. Get a sketchbook or old papers or whatever you can draw on, and draw. Sketch the top-view layout first. As you sketch, you'll see things like long sightlines, and you can decide where to break them up. It's a lot easier to erase a pencil line than it is to rearrange geometry in Hammer.
Remember: Practice safe design. Use a concept.