I think your applying your personal health bias to the fitness of a teenager who is probably in the physical prime of their life. Unless your a long distance runner, it probably not comparable.

In highschool I'd run 10-15km cross country a day, bike 6km round trip to school, run up/down the 5 story buildings all day while wearing shoes that each weighed 2.5kg (yes I weighed them, the model was called "dlthe destructor" my parents were sick of buying new shoes) play a 2 rounds of basketball/soccer at lunch, have a 90minute pe class, then get home and spend 2hours at soccer practice. Admittedly that's probably one of the more full on days, but today my arch nemesis is a slight incline. Point is, walking 20-30km over a day is hardly unreasonable for a teenager.