Golden Rule of Training
This sounds obvious, but many runners train as hard as they can, while taking too little rest, or ignoring the warning signs of impending injury. When you plan your training, remember Supercompensation and build in sufficient recovery time. Remember to do Safe Speedwork. Also, be prepared to modify the plan depending on how you respond to the training. If you are not responding to your training, it could be that you are not training enough, or because you are not resting enough. Think carefully to work out which situation you are in.