From the actual audio of the phone call that Zimmerman made to 911:
Zimmerman: "He's running."
911 Dispatcher: "He's running? Which way is he running?"
*sounds of Zimmerman getting out of the truck, and starting to run*
Zimmerman: "He's headed towards the other entrance of the neighborhood"
911 Dispatcher: "Ok, which entrance is that that he's head towards?"
*sounds of Zimmerman continuing to run*
Zimmerman: "The back entrance..."
*more running sounds*
Zimmerman (under his breath): "It's ***ing cold."
911 Dispatcher: "Are you following him?"
911 Dispatcher: "Ok, we don't need you to do that.
*stops running, still breathing heavy*
911 Dispatcher: "Alright sir, What is you name?"
Zimmerman: "George. He ran."
911 Dispatcher: "George, what's your last name?"
*no running or heavy breathing sounds for the remainder of the converstaion*
At this point the 911 dispatcher and Zimmerman have another minute or two of conversation discussing his address, phone number, where George wants to meet the officer when he gets there, directions of how to get there, what address his truck his parcked in front of (George says he doesn't know he has to check), etc. There are no more running sounds for the remainder of the conversation. It sounds as if he is either standing still, or walking slowly. They get off the phone.
The first 911 call reporting a fight occured 2 and a half minutes later. George stated in a walkthrough with the police the next day that after he got off the phone, he started walking back towards his truck. While walking back he encountered Trayvon Martin. Both he, and the girl who was on the phone with Trayvon at the time, said that Trayvon said something first (accounts differ as to what was said) and then Zimmerman said something in response. It was at that point that the phone got cut off, so there is no objective evidence as to how the altercation actually got started.
911 call audio from you tube, video of Zimmerman's walkthrough with the police the next morning, and testimony of Rachel Jeantel