Just to put in a last word for Chenoweth, not that she needs one from me, but Lily Garland is a very demanding role, physically and vocally. Kristin flows through the show so effortlessly that this may be hard to recognize.
Madeline Kahn, 36 at the time, only lasted a month as Lily in 1978. According to her brand new biography, she was "terrified" of the score and of missing the high notes and kept missing performances.
Judy Kaye, who replaced Madelyn, was only 30.
Chenoweth is 46 going on 47 and in 2012 suffered horrendous injuries when a light fixture fell, hitting her in the face and driving her head into the ground. She suffered a fractured skull and hip and rib injuries for which she was still receiving rehabilitation last year.
In addition she suffers from Meniere's Disease and bronchitis.
To my knowledge she only missed two performances; perhaps it was one or two more than that.
I'm sure that there were days that she could not give her "A" effort, but Florence Birdwell taught that if hurting you still go on and perform as well as you are able.
Here is an interview with Elaine Paige, if you haven't already heard it, on the night before the Tony Awards where she confirms talk of transferring 20th Century to The West End and her intention to go to London with the show if it happens.
https://www.broadwayworld.com/article/Kristin-Chenoweth-Talks-Tonys-ON-THE-TWENTIETH-CENTURY-West-End-Transfer-With-Elaine-Paige-20150613