Be sure to chose your methods of training to meet your needs and goals. I train for "strength and power." I love lifting heavy stuff, sometimes even competing in powerlifting and other strength events. I love martial arts, gymnastics, and bursts of quick speed and coordination. This means that I train to improve my performance in these examples, using heavy weights, high speeds, and multi-directional functional movements.
70% of my training involves low reps (3-5), low sets (2-5), and long rest periods (2-5 minutes) between sets. The other 30% of my training involves the opposite methods, with lighter weight, higher reps (10-20), moderate sets (3-5), and short rest periods (30-60 seconds) between sets. However, this 30% of my training is mainly to improve my general fitness, muscular endurance, and cardiovascular health. It has little effect on improving my performance in strength and power events.
This also means that if you are a triathalete, swimmer, runner, cyclist, etc, you would not be spending much of your time training like me. Perhaps things would be flipped, with 70% of your training being that of muscular endurance and cardio, and 30% being geared toward building a larger strength base or correcting strength imbalances across joints. If you are training for overall fitness and health, you get to choose how you split up your training, but I would suggest an even 50% strength/ power and 50% endurance/ cardio. Too often, people go overboard with the cardio, and miss out on all the wonderful benefits of greater muscle mass, strength, and metabolic demands.
Most of you should be training smarter....not harder....