Yeah and it's funny you say that because I just talked to Clint Oldenburg, the Madden producer, on Twitter about that.
It is tap LT once to pull the passing icons down and become a runner.
Then tap LT again to go back to being a QB.
That information is not listed anywhere in the game or on the game controls screen so I tweeted to Clint "please add this as it is a really cool function and people need to know how to use it".
Who knows if they'll ever add it though..but that's how you do it.