Finding yourself in a situation that is fast-paced, and physically overwhelming. Often that feeling is heighted by a sense of mass confusion, and personal ineptness. Term was first coined to describe the "D-Day" invasion of World War 2. When Allied ships inflitrated Nazi shores, and infantry troops literally had to "hit the ground running", to keep from getting shot.
"I started my new job today. It was so busy, I had to hit the ground running, and had no time to get any training in."