Ever Had "Run Away" Legs?

I have had a frightening experience while out walking. My legs and feet started to speed up. This was not voluntary. I could not slow down and actually continued to go faster. This was the second time it happened and the worst ,time. Our roads are rural- no sidewalks and gravel shoulders. I was aware I was weaving back and forth and I'm sure I looked drunk. I knew I had to stop before I fell face first on the gravel. My solution ? I was approaching a stand alone mailbox. I ran into it and held on for dear life. I called my husband and the woman across the street came . I got home exhausted and worried. What the heck was going on.? Has anyone had a similar experience??