In The Last Lecture, what was Randy talking about when he said that he was stronger than anyone else?

In The Last Lecture, Randy argues that despite the cancerous tumors riddling his body, he is in better shape than most of the people in the audience.

Randy's assertion is made at the start of the lecture. He wants to address "the elephant in the room."  People are there to listen to his "last lecture."  He knows the pressing issue for the audience is that they are listening to someone dying: "This is what it is."  However, Randy does not want the audience to feel bad for him.  He believes their pity will get in the way of his message.  In a way to both allay their fears and generate some much needed humor after seeing CT scans of his many tumors, Randy claims that he is stronger than most of the people in the audience:  “I am in phenomenally good health right now...  I mean, the greatest thing of cognitive dissonance you will ever see is that I am in really good shape. In fact, I am in better shape than most of you.”

Randy proves his strength by dropping to the floor and giving successive pushups.  This makes the audience laugh and applaud.  Randy's assertion of being stronger and in better shape than the audience members brings out the "cognitive dissonance" that underscores Randy's lecture.  There is always more than what we see on the surface.  For example, while Randy might be dying of cancer, he is in the best shape of his life.  While we might normally think that a "last lecture" would be morose and sad, what Randy delivers is the opposite.  In this way, we might think that Randy, being a terminal cancer patient, is weak and debilitated.  However, with his affirmation of strength, he proves there is always more than what we perceive.