There is lots of advice available for runners - some of them conflicting and often confusing. Professor Tim Noakes, a well-respected sports scientist from South Africa, has provided the following guidelines, compiled from a number of sources:

During any one year, train for 10 months and take a holiday from running for two.
Don't be ashamed to walk during any training run. Maximum benefit on any run is achieved by training at between 60 and 90 percent of your MHR (Maximum Heart Rate).
The important element of training is the amount of time you spend running each week, not the speed at which you run or the distance you cover.
All training schedules should be treated as guidelines, not concrete regulations. Learn to listen to your body so you can judge your own most effective schedule.
Always allow time for a hard run's micro-damage to your body to repair and for your energy stocks to replenish.
Find your happy training medium between the two adages 'No pain, no gain' and 'Train without strain'.
Don't race during training runs. Run races of over 10 miles only infrequently.
Running trains only the legs, not the upper body. Adding an activity such as swimming to your running programme is a good idea.
Never over train. Watch out for sluggishness and lethargy, loss of appetite and increased infection susceptibility.
Where possible work with a coach for encouragement and support.
Success in running involves training the mind as much or more than the body.
Rest before racing.
Keep a training logbook. It will allow you to check exactly what training you did before your best race results.
Everything you do, all day and every day, affects your running ability - nutrition, stress and sleep as well as exercise.
Professor Noakes, your training club, friends or family are not responsible for your training - advice is often given freely. It's up to you how you use it.