My friend and classmate David Berger has allowed me to share his take:
Alright, this keeps coming up: public health types saying stupidly pessimistic things about the number of hours you have to burn off x amount of calories. Friends, it's easy to deceive yourself about how many calories you actually burn doing cardio. And then there's a whole bunch of people--treadmill manufacturers, for example--who want to inflate the numbers. But this trend of public health know-it-alls using the most pessimistic calculations needs to stop. It's just wrong. These people convinced teenagers that it would take 50 minutes of running to burn off one soda. They must be targeting non-runners.
Actually, who they are targeting is baffling. They base their calculations on the activity-energy equivalents for a 110 pound fifteen year old. Nowhere do they indicate pace, although when I use the calculator on runner's world to give a 110 pound a 15 min/mile pace (a pace walkers in comfortable clothing can manage), it gives me 277 calories for 50 minutes.
Let's do a real calculation. If you are less worried about 110 pounders drinking soda, try a 200 pounder. Let's give the same walking pace of 15 min/mile, and keep it at 50 minutes. 504 calories. Alternately, that's 25 minutes to work off a soda.
Or, suppose you expect someone to aspire to something, and someone reading this public service message to know the difference between walking and running, and to be able to determine whether they can maintain a running pace for 50 minutes. I won't even make them a good runner, just a 12:30 min/mile, which someone starting out can manage in most cases. The same 50 minutes goes up to 605 calories. Granted 50 minutes might be much for someone starting out, but then they only need 21 minutes to manage a 250 calorie soda.
Now, calories/hour will be lower if you weigh less than 200 pounds. But then you will probably be able to run faster than 12:30. I understand this push society is making overall: there's too much false hope in the ability of exercise to compensate for constantly immoderate caloric choices, and there's too much acceptance of empty nutrition (like soda) as a source of calories. But if the next heavy-handed social tactic is outright lying, can we please just not?