To get the most out of your training, it's important to customize it to the needs and capabilities of your unique body. There are several important variables to consider in this regard. Among the most important is your age.
Stage One: Young Runners
Runners in their late teens and early 20s have a much greater margin for error in their training than older runners. Any type of training that does not injure them or result in chronic overtraining fatigue will improve their fitness and performance.
Like all runners, they will derive the greatest benefit from the most appropriate training, but young runners are typically able to get a greater benefit from less specific and even borderline irrelevant training than older runners. Runners of every age, however, should train as appropriately as possible.
Due to their strong recovery capacity, young runners generally do not need to train as lightly on their easy days as older runners. If you're between 15 and 25 years old, you can probably train at least moderately hard every time you run, assuming you have an active background. You can reserve the very slow recovery runs that older runners depend on for those rare days when you feel especially lousy.
By the same token, young runners cannot expect to train quite as hard in their demanding workouts as more experienced runners in their late 20s and early 30s. It takes years of consistent training to reach the point where you are able to tackle the toughest workouts you will ever do, and no runner under the age of 26 has this foundation. It's OK to do some gut-busting training sessions when you're young, but it may be wise to hold back a little in planning these workouts. Take the long view, and draw patience from the knowledge that those epic track sessions and long threshold runs you dream of are waiting for you just a few seasons down the road.
Stage Two: Experienced Runners
The period between the mid-to late 20s and the mid-to late 30s constitutes another stage in a runner's development, and it requires a slightly different approach to training. At this time, your capacity to recover from hard training is decreasing, so you need to take your easy days seriously to fully absorb your most recent hard session--and to get you ready for the next one.
On the other hand, if you've been training consistently for at least a few years and your body has responded to past training, you can handle harder workouts. Your muscles, bones, joints and neuroendocrine system are now far better designed for hard training than they were when you were younger.
Stage Three: Masters Runners
Somewhere between the ages of 40 and 45, a third stage in the development of a runner begins. In this stage, it is extremely important to get the maximum bang for your buck from workouts. Most of your hard workouts should be very race-specific. There's not much to be gained from general training anymore. Your body has adapted to so many different training stimuli over the years that there are few remaining stimuli that are capable of triggering fresh fitness adaptations, so choose very carefully.
You're a long way from those early days when virtually anything that got your heart rate up made you a better runner. On the positive side, your many years of training and racing have taught you many things about yourself as a runner--and you can use this knowledge to select just the right alterations to your training approach that will keep you moving forward.
There should be few or no "junk miles" in your training after age 40. Accumulated wear and tear on your body has probably already put you past the point of being able to set new PRs, and continuing to fill your schedule with a lot of basic aerobic runs will do more harm, in the form of additional wear and tear, than it will do good. You're better off doing just three or four focused sessions per week in the muscle training, threshold training and specific endurance categories--and strength training or cross-training on the other days.
Masters runners also need to be careful with speed work (training at 5K race pace and faster). The muscles and connective tissues have lost elasticity by this age, making them more susceptible to strain during high-speed running. As a masters runner you should do as much speed work as you can safely handle that is relevant to your event goals, but be prepared to find that you cannot safely handle much. Adjusting your speed work for your age does not mean doing less. You can move some or all of it onto hills, which reduce tissue strain at higher intensities, or you can simply slow it down a little--for example, replacing 1500m-pace intervals with 3000m-pace intervals.
For better and for worse, your body never ceases to change through aging. Your approach to training should reflect this fact by evolving from year to year in appropriate ways. Use the guidelines presented above to continue developing as a runner--and to stay strong through the years.