Retreat and how you retreat is a tactical option dictated by circumstance.
Sometimes it is better to stand and fight, sometimes its better to turn and run, sometimes its better to back peddle and hope the circumstances change. Other times just give up what the person wants.
Look at it this way. You nubile and young are faced with a lumbering ox without a projectile weapon - what would you do?
On the other hand you are now faced with someone with your build, a knife and the desire to use it. Like you alluded - turning your back might be the last silly thing you do.
And now to my favourite running away joke.
Two guys in the woods come across an enraged bear.
Person 1: Run its a bear!!!
after a few minutes of persuit.
Person 1: Gasp gasp we'll never outrun the bear.
Person 2: What do you mean - I only have to outrun you.