Hahahaha, too right, Omen

Now this is based on what I have seen. I've only played 5 seconds of Halo (there was the demo of MP next door, so when the line went down, I played that), but the environments are heaps larger in Halo, especially the outdoor ones. They are much larger than MP. Can anyone tell me how you get to other areas in Halo?
Like, In MP, they are all connected by that elevator. In Halo, the whole 'world' is the inside of a huge ring in outer space. Can you you just walk around the whole ring and end up where you started? Or is it stage-based like MP?
And form what i remember someone saying (though I forget), the reason why there's a Halo 2 is cause the humans/main character/whatever failed in the mission/saving the world/whatever in the first game.