I think it assumes you run regularly and are therefore fit for running. The same way if you lie in bed for a month you're not going to be doing 20 hour hikes e

I think it assumes you run regularly and are therefore fit for running. The same way if you lie in bed for a month you're not going to be doing 20 hour hikes either. Fitness isn't a one-dimensional thing -- our muscles adapt efficiently to our specific activities and only our specific activities. So comparing the efficiency of walking and running only makes sense for someone who is already well-adapted to both.