The most important thing to running is listening to your body. Whether you are training, getting in shape, jogging, or even a casual walker listening to your body is the best thing you can do. Your body tells when enough is enough and by not back offing you put yourself at risk for injury. When you feel a small ache but choose to ignore it, it can quickly escalate into a more serious injury and it may even last for several months or even years. Even fatigued muscles should be taken seriously. When muscles are tired they are more prone to rips and tears.
If you are out on a run and you begin to feel a slight back in your body the best thing you can do is stop and walk. Do not keep pushing because you will hit a moment of pushing too far. You are better off stopping and walking home and consulting a doctor before it escalates into something worse. Depending on the pain you may want to stretch and ice the pain and take a few days rest. What most people don't realize is that rest is just as important to your body as training. If you injure yourself you can be out for a long time recovering and that is all lost time you could have been training. A few days rest is a lot better than having to take a few months off.
You also need to listen to fatigue. If your muscles are tired, heavy and it's hard for you to run at your normal effort it is your body telling you you need some rest. If you continue running on fatigued muscle you are more likely to tear a muscle and torn muscles take months or even years to fully recover. If your muscles are continually becoming fatigued try taking ice baths after your runs and stretching. But try stretching before the ice bath when your muscles are still warm. You may also want to try cutting your mileage down and after you find a point where you are not fatigued continually you can pump your mileage up increasing only 10% every week.
Being aware of your body and knowing when you are pushing yourself too hard will put you at a huge advantage when training. It's a hard concept to understand because most people believe the harder you train the faster you get, but, that isn't always true. Know your body, listen to it and you will be a healthier and safer runner.
