As fast as your legs can carry you
Definition 1 of 1
Pro Tip 1/3Maximum Speed
It expresses the fastest possible speed someone can achieve by running. 
Escape the fire as fast as your legs can carry you!
Pro Tip 2/3Urgent Situations
Used to describe situations where one needs to move quickly to avoid danger or reach a destination urgently. 
Grab your passport and go as fast as your legs can carry you to the embassy.
Pro Tip 3/3Colloquial Humor
Often used humorously to exaggerate the urgency or the speaker's earnestness in telling someone to hurry. 
Run as fast as your legs can carry you, or you'll miss out on the free pizza!