Skateboarding may make you feel more fit externally, and may in some cases get the heart going faster than it might have normally been without skateboarding.
However, to fully put into shape the body needs to be engaged with rigorous activities over time that also work on building different muscles in different ways and shifting metabolism so as to release fat reserves.
Skateboarders are often encouraged to do things like this when they first start out and can negotiate stairs easily: trying parkour (free running) or rock-climbing for an hour each week.
Additionally, people are sometimes led astray by believing skateboarding is a sport because of its inclusion in extreme sports competitions. In fact, skating falls under the category of action sports.