I think it's important to note that a big part of free soloing is the mental game, and no amount of rehearsal on a rope can fully prepare you for that. Alex had free soloed thousands of pitches of rock, at vary degrees of difficulty before taking on El Cap.
What impresses me most about Alex and other free soloists isn't so much their ability to keep their head together while climbing, but their ability to keep their head together while lying awake in bed at night, thinking about what they just did, and what they plan to do again tomorrow.
What impresses me most about Alex and other free soloists isn't so much their ability to keep their head together while climbing, but their ability to keep their head together while lying awake in bed at night, thinking about what they just did, and what they plan to do again tomorrow.