Form and Challenge : Finding the Correct Balance

There is a precipice that we must try to balance on with weight lifting, and all other forms of exercise. What is this hilltop that we must straddle? It's finding the right amount for you. With weight lifting it is being able to have good form while challenging yourself. If you have good form, but feel that you could go on doing reps for hours, then the weight is probably not heavy enough. If you feel exhausted at the end of three sets, but can only make it through by sacrificing your form, then you are trying to lift too heavy a weight. The right amount of weight is the one that can be performed for three sets with good form and feel as though you couldn't lift it any more by the end of the third set.
Running has a similar balance. How far can you run and how fast without sacrificing your form? If you are able to keep good form, but feel that it is not challenging, you are probably not running far enough or fast enough. If you can run a long distance at a good clip, but your form is very sloppy, you are putting yourself at risk for injuries. The right way to run for best results is to run at a challenging pace and distance that you can perform with the correct form.
The balance of form and challenge is also seen with yoga. If you can attain an asana, but only after sacrificing your breathing rhythm you are trying to do too much. On the other hand if you can keep the rhythm of your breath, but don't feel any stretch, you are not challenging yourself enough. The right balance is the one which is challenging for you, but not so challenging that you can't breathe properly. It's never a good idea to hold your breath while exercising, it may lead to the embarrassment of passing out.
Healthy moderation in exercise is finding the balance of form and challenge. You know your body better than anyone and can fine tune this over time until it is easy because it becomes habitual. Making a concerted effort to balance these two elements will lead to a healthier body with less injuries in the long run.

Finding Your Weaknesses

One of the keys of achieving fitness through moderation is variation. The wider you expand your scope of possibilities for physical activities, the more well rounded your fitness will be. I write this after a weekend of playing woodsball, hiking the woods, crawling through brush, doing what you can to shoot paint on other people before they get you. Once I drove home I realized that I achieved quite a workout to my glutes and quads. I WAS SORE. From the lower half of my back to my hamstrings. Since this was the first time in the year I had played woodsball I wasn't all that surprised, but since I am fairly active I thought I would be in better shape for paintball. If I had varied my exercises a little more I may not have been so sore.
Now that I realize that hiking an incline at a fast pace, and crawling through the brush are my weaknesses, what should I do? Find exercises that exploit these weaknesses until they become strong. Part of moderation is not overdoing it though, and so I won't work on it while they are still sore.
Varying physical activities is one way to find weaknesses in your fitness. After you can find your weaknesses, you can make them stronger. Knowing yourself can play an important part in finding your weaknesses and turning them into strengths. Generally, if you are in tune with your body you can tell the difference between muscle soreness from working out, a torn or sprained tendon, or something more serious. If you have any questions about a pain, you should seek medical attention.
Variation is a useful tool in gaining fitness through moderation. It can highlight your weaknesses, and show you areas that have room for improvement. Once you know what areas you can improve, you can work to make them as strong as other areas of your fitness.