It won't take long to adjust...
But unless you are practicing on Halo 2 LIVE....then don't bother.
Campaign is COMPLETELY different than LIVE play...
Yes, I'm aware of that. Multiplayer alone is completely different than campaign.
But just to get in the habit of things, I'll probably play campaign for a week or two. Then I'll invite my friends over so that I can kick their asses. >_> Hopefully.
And then, THEN I'll get a 360, H3, and LIVE so that I can get my butt handed to me by you guys.