Training your body to do something is actually a fairly straightforward concept: you must train hard so that your body adapts to the training, and you must keep giving it progressively harder exercises to do, so that it becomes increasingly stronger.
At the same time, you must also recover let your body recover in between workouts. If you train hard and never give your body a chance to build itself back up, you'll simply wear yourself down to a crisp.
It's a simple concept, but one which a lot of people seem to get wrong.
What you're doing when you're exercising is actually tearing your muscles down. After you've torn them down, those muscles, with proper nutrition and rest, then rebuild themselves in ways that next time it will perform that exercise better. But without that recovery period, you'll just continually tear your muscles down and not give them a chance to recover.
Most people do either too little or too much. People who aren't into exercise just let themselves go, and wind up with minimal muscle and maximal fat. And people who are addicted to their exercise tend to overdo it, and get injured that way. Injuries tend to happen when someone doesn't give himself enough recovery time. And that period of recovery time needs to be longer as we get older.
The average jogger will run something like three miles a day, every day, at eight minute pace. What he is effectively training himself to do is run three miles a day at eight minute pace. Period. He'd be better off running five miles, every other day. He'd not only be training his body to run further, and therefore be stronger, but he'd be allowing it to recover in between runs.
He'd be even better off if he varied his exercise more, so his body adapted to different types of stress. If he ran sprints and did calisthenics Monday, took Tuesday off, ran long distance Wednesday, took Thursday off, then swam on Friday, he'd find himself feeling much stronger than if he just ran three miles every day. He'd have more speed and explosive power, more endurance, more looseness to his muscles, and healthier joints. He'd also look a lot better.
Look at the people you know who run daily but do nothing else. They mostly look like the before picture in the old Charles Atlas ads. Plus they develop a certain brittleness to their bodies. They'd be much better off varying their exercises, and incorporating yoga and other upper body exercises. With the advent of Pilates, body sculpting classes, kettlebells, yoga, etc, fewer people are getting it wrong than twenty years ago; but many still are.
A former Mr. Olympia, Dorian Yates, trained each muscle group exactly once a week. He said if he went hard, that was all he needed to do. And he understood the damage that overtraining can do. Arnold Schwarzenegger, back in his Mr. Olympia days, once purposely sabotaged up and comer Lou Ferrigno by telling him that he himself trained with massive amounts of reps -- which wasn't true. (Not unlike the way the California legislature is now sabotaging Schwarzenegger.)
Think of the sprinters in both swimming and running. Most of them are basically lazy But as a result, many of them have more explosive power. Sprinters tend to look like studly, distance athletes geeky. Some of this is genetics, but some of it is the nature of their training. Be dedicated, work hard, turn yourself into a wimp.
Another concept that many people tend to lose sight of is specificity of training. If you're training to swim a 100 yard freestyle in under 50 seconds, doing endless repeats of 100 free's (or multiples of 100 free's) at 1:10 pace will do you next to no good, and it could even harm you. What you should be doing is repeats of 50 freestyles in as close to 25 seconds as you can get. These stressful days should be interspersed with easy days, which might include 100 yard freestyles at 1:10 pace. But if all you do is swim long distances slowly, your sprinting will never improve. (I use swimming as an example because serious track athletes tend not to get this concept wrong as often.)
My apologies for this boring post, but it's advice some would do well to take.
I suppose I could heed it myself. Last night my son mocked me by pulling his t-shirt over his chin (to represent my weak chin) and whining, "I can't get my books published." When I told him that he shouldn't be so hurtful to me during my formative years, he replied, "Yeah, formative -- formative of that spare tire around your stomach."