Train Smart, Not Hard
When I was 18 years old I used to squat until I puked (I have a few past training partners that can back me up on this). The volume and intensity I used to work at was insane. Not so much these days and not because I'm lazy.
If you want to build muscle, be lean and healthy you don't need to kill yourself to get there. Don't get me wrong, you still need to push hard in the gym but as you get older there's really no need to push yourself to a crazy level like I once did.
If you want to get lean, build muscle and be healthy without injuring yourself or training so hard you can't function the rest of the day follow this rule; When you work out, always evaluate the risk (injury) to reward ratio for your goal. One of the biggest mistakes I see even advanced people make at fitness clubs is using too much weight for their exercises. Ask yourself if it's worth squatting 400lbs over the possibility of damaging your knees. Ask yourself if there's another way of achieving the desired outcome. Now if your goal is to be as strong as possible then you may not have a choice and will simply have to take precaution in your technique.
So what is the smart way to train to build muscle, get lean and be healthy?
It starts with adopting the lifestyle and not just making this a fad. Clark says it best in this video "The best way to get in shape is to stay in shape". Second, nutrition is about 70% of the battle yet many people think they can just workout and eat whatever they want whenever they want. If you're one of those genetically gifted people that can do that, let's see if you can still do that in 5 or 10 years from now. No matter what, nutrition should be at the forefront of your training program. Drop the hour long workouts and stick to 30 minutes of training a day max. Use moderate/heavy weight and try not to sacrifice form over the weight lifted. Work the muscle with full range of motion and focus on proper recovering.
With the combination of a great nutrition and a well structured training program, you will not only look good you will feel good too!