After Lex took another 100 steps, he developed somewhat of a rhythm. The rhythm helped reduce the strain on his mind which was focused on even the subtlest movements he made with his body during this time. The hyper focused state he was in was also one of the reasons he had failed to notice his growing and evolving aura.
What he did notice, however, was that while he had expected each progressive step to grow harder, sometimes it would get easier instead. He could not understand why the strain would reduce, and he didn't have the mental capacity to spare to analyze such a thing.
Now that the strain on his mind had eased up a little, he could focus on the next thing, which was to speed up. He had to be especially careful with this as he did not want a recurrence of him tripping and injuring himself. His task was already hard enough without accumulating more injured joints.