Let's assume he has a backpack if he's going to walk. Let's assume too that he's lucky and it's not raining. But especially with that weight on his back, downhill -- and it's all downhill -- will not be fun on his knees.
And I assume we're only talking about walking down to Tung Chung. Because of the highway maze, it'd be near-impossible to walk out to the airport from Tung Chung. At least I've never seen a soul doing it.