Avoid backpeddling when locked on

Solution 1:

No, unfortunately you need to cancel the lock-on to be able to do this.

When you're locked on to an enemy, all of your movement becomes relative to that enemy. Up moves you towards it, Down moves you away -- backpedaling, as you know -- and Left/Right rotate you around the enemy in a circle. If you want your character to spin around and run directly away from the enemy, you need to be using Legacy controls and not be actively locked on to anything.

Your situation isn't terribly uncommon, though, so you might be able to use some of the tips in those various discussion threads to get the best of both worlds, to a degree.

(Disclaimer: I haven't played the game in around 6 months, so maybe they've added some way to do this in that time, but a quick search doesn't seem to indicate such.)

Solution 2:

While locked on: Stafing does a circle strafe; Backing does a backpedal. To avoid locking on, uncheck this setting:

Character Settings ->
  Control Settings (red button on left) ->
    Target (top tab) ->
      Target Settings -> Automatically lock on target when initiating auto-attack.

By disabling the lock on, your character will move the same in and out of combat.

When backing up: Standard movement backpedals; Legacy movement turns and run away. To change to this setting:

Character Settings ->
  Control Settings (red button on left) ->
    General (top tab) ->
      Movement Settings -> Legacy Type (Camera-based).

I play with lock on disabled and the standard movement setting. Coming from World of Warcraft where backing always backpedals, I naturally strafe sideways out of circle aoe's.