Not a gym rat, but I do a lot of exercise. I would describe myself as fit and am quite strong, but fairly lean compared to people who lift a lot of weights. Sometimes I notice all of a sudden that a new muscle has developed, that I have never felt before, which is a really cool feeling. I'm tall, with a solid build.
If I get more time, I will begin lifting weights so that I can gain the extra muscle mass, but I also quite like the type of functional strength and lean physique you get from exercise that doesn't involve weights. It produces a very different type of muscle to what weights do and actually changes the way your body processes energy, making it more efficent. The body actually becomes more capable of absorbing oxygen and can use it more effectively. When the body runs out of oxygen to burn due to strenuous exercise, it starts burning other things and this produces lactic acid. Lactic acid inhibits the contraction of muscles causing you to fatigue and start to feel weaker. When you become more fit, you start to have physiological changes, the body produces less lactic acid because it is absorbing more oxygen and becomes capable of removing lactic acid from the muscles more quickly, so that you gain more endurance. Even a single, very hard exercise session, can produce significant physiological changes in the body, so it can be really cool what it can do if you train at high intensity for long periods of time.
I use my shoulders a lot when I exercise, and put them through thousands of repetitions per session: 2,000 to 4,000 reps, three times a week. The shoulders are a very vulnerable, free floating joint, so I need to start lifting weights so that I can build a strong structure of muscle around them, in order to protect them from injury.