In the scenario you're painting I'd keep walking calmly. Running would trigger the hunt reflex in them.
If they actually physically hit me though I'd grab the heaviest and hardest thing I'd have on me and try to give the leader of the group brain damage before running away like a coward.
Luckily I live far away form any area I can imagine such a thing happening.
These questions stump me. I'm currently 22 and have never been in a fight with anyone and I live in a somewhat notoriously 'bad' area. The way I figure it is if I don't act like a dick nor hang around areas that attract dicks then I won't need to be in this situation.
Though there was one time I did get mugged. When I guy pulls a knife on you and asks for your phone you give that motherfucker your phone.