Can I Ready an action to run away?

Question

This is a follow-up to Can I Ready an action to Disengage?

It looks to me that the 5E RAW does not have a simple way to declare a behaviour like this:

If anything nasty comes towards me, I run in the opposite direction!

That is due to how the Dash action is worded as an increase in movement allocation for your turn, and not as an additional segment of movement in its own right:

When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers. With a speed of 30 feet, for example, you can move up to 60 feet on your turn if you dash.

So, importantly, Dash does not make you move, instead it adds to your "movement allowance" for the turn.

In addition the movement rules say nothing about movement except during your own turn (emphasis mine):

On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here.

There are no rules about having or using movement on other turns than your own. Is this an oversight?

Answer 1

Yes you can.

The Ready action is what you're looking for. It's explained on p. 192 of the Player's Handbook:

First, you decide what perceivable circumstance will trigger your reaction.

For you this would be, "If anything nasty comes towards me".

Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

So your response would be, "I run in the opposite direction!"

So you use your action Ready to run and then, when a monster approaches you, you may use your reaction to move away. Generally you won't also be able to Dash since that would require another action.

Answer 2

You can't use Dash. But you can move your speed on your turn, and then move your speed again as part of the Ready action. Here's the text:

Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it (Basic Rules, p. 75).

So, if you move your speed on your turn, and then ready an action, you can use that readied action to move your speed. This is in effect the equivalent of Dash, but it does not use Dash as its mechanic.