Lol! Skateboarding no more efficient than walking! You have obviously never been on a skateboard. I get out at least once a week and it is a killer cardio workout. In addition to the fact it is actually fun! I can't stand sitting in the gym doing cardio, so between this, hiking and the bike once and a while i get plenty. I have never felt better since starting back. I would suggest buying smaller wheels under 55 mm. And make sure you push and ride both ways to prevent imbalances. Doing tricks also increases intensity, but keep them simple to prevent risk of injury. No nollie tre flips down the ten sets. Besides the cardio aspect I have never been more sore in the adductors, my calves have improved greatly, and my proprioceptive balance has never been better. It seems to have even improveD my golf swing, likely due to the rotational moves like 180 Ollies etc. you could also do things like wearing long sleeves and pants to increase the thermogenic effect.
Oh and Mike mattarazo rode a skateboard. He said riding it to the gym helped his conditioning and calve development.