Since they have all had physicals and there are always medical personnel in attendance, I think the walk wasn't too much. The girl that got sun stroke was running at the beginning instead of walking and determined to win. She'd have been better off if she had walked.
They also don't exercise that hard all the time. You have to take into account there is a lot of hype for the cameras.
HW 356 pounds - CW 135 - GW 137
Life isn't about waiting for the storm to pass. It's about learning to dance in the rain.