Well, that's practically the whole story, finally.
Every time the runner's front foot plants, there's a huge amount of kinetic energy lost. This dominates all other effects. What happens when the skater's front foot lands? It's night and day.
"airborne" is irrelevant, it's the lost FORWARD momentum that's pertinent. Interestingly, this effect is camouflaged, and reduced, by the body's flexibility; i.e. the ankle and knee joints. Focus on his center of mass, at his navel, and there's only a small loss of velocity.
If the body was stiff, the Tin Man, the effect would be obvious. But as the body shifts shape, on each step, the center of mass also shifts. Still, there's a big loss of energy on each step.
Walking is a red herring. It's easy to believe that, at 4 mph, walking is more efficient than cycling. That isn't the question. Obviously, at low speed, there's less kinetic energy lost.
It's a comparison of athletic performance. A biker can easily do
20 mph. What happens when a runner attempts that?