1.when you pass someone going the opposite direction, you can raise your hand, nod or mumble a little something once. You don't have to keep acknowledging them later if you see them.
2.runners do not have to "warn" other runners,walkers or cyclists if they are approaching from behind. This is too much like cyclists yelling "right" or "back".If you scare the crap out of them , then tough shit , they shoulda been paying more attention.