How does defense work if I dual-wield, or use a bow? Just run out of reach?
Pretty much, yea. With the bow, you can eventually get a perk to smack things with the bow to make it easier to keep anything close off of you. Dual wielding, when you level it up enough, you strike things so fast and it does so much damage, you don't really need to block, especially since it staggers them when you dual power attack.
I still prefer to use it as a secondary means of attack rather than my primary since mages will mess you up on higher levels if you're not skilled in blocking. Dragons too. There's different ways you can get around that though. Enchanting for magic resistance, or the shout "Become Ethereal" which makes you immune to damage and look like a ghost for a time. Makes it easier to get close to your opponents and put the hurt on them quickly.