Let’s say you wanted to learn to play the piano. You could go buy a piano, set it in your living room and hammer away, until everyone within earshot is ready to come at you with a cleaver. It is possible that after countless hours of committed practice (assuming frustration and cleavers don’t deter you), you could become a tolerable pianist.
Or, instead of spending months and years re-inventing the wheel, you could find a teacher who can help you do things the right way, the first time.
The same can be said about getting in shape. There’s a right and many wrong ways to do just about every exercise. There are certain exercises, intervals, rest periods, and diets that work best for achieving different goals. While the fitness industry seems flooded with questionable information, there have been decades of solid research done on these topics.
A major difference between training and playing piano though is that if you play the piano wrong, you might stay terrible, but you’re never going to get hurt. We all know that exercise is important; doing it right is even more important. So whatever your goal is, find someone qualified to help you out!