Why was it not necessary to break Jesus' legs during crucifixion? Jesus was put to death by crucifixion, between two "real" criminals, on a Friday.

Because the Jews wanted the execution to be over by the Sabbath, they had the Roman soldiers "speed up" the process by breaking the legs of the two (still-living) criminals. But they didn't need to do this to Jesus because he was already dead, meaning that his body was "whole" when it was buried.

Why did Jesus die before the other two, who "survived" him? Was it because he was in worse shape than them? (Of note is the fact the he fainted while carrying the cross.)
The scripture says that Jesus "gave up his spirit" in Mt 27:50 and Jn 19:30 (it is implied in Luke). The description in John is particularly enlightening because it says that Jesus got a drink and gave up his spirit because he knew "that everything had now been finished, and so that scripture would be fulfilled". Jesus chose to give up his life at that moment, and he chose precisely the right moment in order to fulfill prophecy. It was not a matter of fitness or being in shape, but rather fulfilling God's will. This is further reinforced by the fact that before letting go of his life he said, "It is finished".

From a physiological perspective, it's also reasonable that Jesus would die faster because of what he suffered prior to crucifixion. He was beaten (Mt 26:67-68), flogged (Mt 27:26), given a crown of thorns (Mt 27:28-29), and struck over the head repeatedly (Mt 27:30).