I do know that this boy (according to the news report) was much smaller than the example you gave. 4 foot 2 inches tall is the size of an 8 or 9 year old. If the kid weighed over 100 pounds at that height, he would be considered fat/obese.
Everyone does have different personalities, but that doesn't change how excessive this punishment is given the information we have. The father didn't say the kid was being punished for being mouthy or abusive or getting into trouble at school/in the neighborhood or other things that could lead a person to believe the kid was troublemaker. No, the reason given was for watching too many videos (there is no reason this could not have been dealt with by taking away all access to electronics in the house), not doing homework (generally involves parents actually getting involved, I still don't understand how a parent could get a child to do that much manual labor but not stand there over the kid's shoulder while he did his homework, which likely wasn't even attempted), and not doing chores (so give more chores or heighten the expectations). Even if the things I gave were tried (one of them, unless the kid is really good at hacking, can be easily done), this punishment would still be excessive, in my opinion. And it is pointless.
And it could cause physical issues. Do you think it isn't warm/hot in Georgia in May? I'm seeing about 75-87 F for this past weekend (which is when it sounds like this happened). If the exercise is something the kid is not accustomed to, the level could easily cause issue with muscle strains/sprains, especially given the amount of weight the teen was carrying for the average amount of time (it would take about an hour, maybe a little more to walk three miles carrying weight).
Obviously we need more information here, but what we have is enough for me to say right now that I find it excessive and even cruel. (I find it cruel for any child/teen to be punished with pointless activities. It seems to be done more for some morbid satisfaction of the punisher or just plain laziness rather than actually being used to help the kid, teach the kid.