This was brought up once before and I have extreme doubts.[You must be registered and logged in to see this link.]
First, I would want to point out that Mr. Thorne identified the person(s) running past the gym as being on the "outside" (based on the gym teacher's report). We can't know for sure if he meant outside the school or outside the gym but that is what he said when he told police that the shadows were going "past the gym."
Next, one minute and twenty seconds (assuming Lanza could do it this quickly) may seem like a short amount of time but the entire shooting was little more than five minutes long. Did Adam shoot all 26 people in 3 and a half minutes in three different locations? All of his confirmed movements were through the lobby and down the opposite hall. All witness statements suggest that he shot people in the lobby, entered the main office (shooting no one), and then proceeded down the hallway to the classrooms where he killed. No one noted him moving in any other direction. People who saw Adam's movements seemed to suggest that he was walking, not running at any time. Evidence also suggests that he had some issues with his guns jamming as well, which would also slow him down as he killed and committed suicide.
Assuming that the runner was inside, I think it is possible that the person seen running was the same person who was seeking shelter in the art room but I can't see Lanza wasting so much time like that. Even if he moved on because he thought that he had killed everyone in those two rooms, why would he run all the way back toward the lobby, turn the corner and only bang on the door of the farthest classroom at the end of the hallway? Doesn't it seem more likely that someone running in that direction was someone running away, as far as possible, from the gunshots that they heard in the front hallway?
Something that would bolster your theory is that fact that he likely entered room 10 first, then went to room 8 and then back to 10. It would make sense if he ran down the gym hallway, saw police arriving, and ran back into the first room that he was certain was unlocked to commit suicide. As far as I know, police have never confirmed that he started in room 10 but witness accounts suggest that shooting was heard moving down the hallway and never doubling back, so I would go with that timeline of movement.
Still, how was Rick Thorne in the gym hallway without running into or seeing Lanza himself if Lanza was really running all over the place? I don't know Mr. Thorne's exact movements but all of his statements suggest that he viewed the lobby from the gym hallway and then stayed close to the gym or in the gym hallway until police arrived. He probably would have seen Adam if Adam came down that hallway.
I still think it's almost certain that this was someone other than Adam.
Here's a compilation of witness statements, just for reference, for anyone interested.[You must be registered and logged in to see this link.]