Exercising is not the same as training. Training is a plan of exercises with a target in mind. Just think about people in the gym...
There you can usually find people who exercise and people who train. There's always someone jumping from one exercise to the other with no plan at all, it's usually the skinny guy. And there's also people who follow a strict routine, with a target in mind; usually the strong dude.
It's obvious that exercising is better than not exercising, as you may get some benefits from it. The skinny guy in the gym may be fitter that the one that never ever goes to the gym. But if you're trying to get better you need a training: this is a plan of exercises devised to reach a target.
So when it comes down to chess tactics, solving random chess puzzles is better than not solving any. But if you want to improve you need a plan ;-)