Call of Duty 4: Modern Warfare

Hey!

So, I just hooked up my XBox 360 as of 15 minutes ago, and I threw in my game. I've passed all the introductory crap, except for the part with Capt. Prices' *barn*? ... I fail everytime because I cannot sprint to the finish line. My menu says to hold the gray button that moves me foward (LS)?, but that does nothing. Any suggestions? Should I click repeatedly or hold it down for awhile? (Both of those I have tried already BTW)

Thanks for the responses :)

Much appreciated.