Question

I run for my life. 

Answer 1

Well this is just an expression and it's expressed as I ran for my life. All this means is that the character is running as fast as the character can, like his life is on the line, thus seeming that the character is "running for his life". The scenario that this expression is used in is when either the character needs to get somewhere, fast, or is actually running away from something. There are rare times where I ran for my life is a literal meaning in which they are running from a threat.

Answer 2

Its just an expression. 

Example: I could eat a cow

It just means that you run as fast as you can. ≈