My gymnast granddaughter Gracie trains 12 hours a week. Three days a week for 4 hours. This past week she put in an extra 4 at the gym "fine-tuning her routine," an optional practice session suggested by her trainer.
I went to her practice the other night and she pretty much was working the whole time. Occasional breaks of a minute or two to change disciplines, wait for the other girl to get off the bars, etc. But mostly she was working the whole time.
It's not the same kind of endurance as running or biking for 4 hours, and not the same type of intensity as triathlon-training 12 hours a week; still, it shows a lot of concentration and determination.
You can see her muscles when she moves her arms and legs.
I wanna grow up to be just like Gracie :-)