Soviet sports scientist N. Yakovlev’s model of training and adaptation. After a workout, there is a recovery period followed by a period of super-compensation. The optimum time for the next workout is at the peak of super-compensation. Too early, and the body is still recovering. Too late and the benefits of the last workout are lost. The intensity of the workout must also be adjusted to achieve optimum training effect indicated by the blue curve. The recovery period will also depend on factors such as nutrition, hydration, and amount of sleep.
How do you know what is too easy and what is too hard? Well, that is where coaching comes in. What I do with my athletes is test them frequently with different test models. Not just 5Ks or time trials. We have workout trials as well that are great indicators of current fitness. The reason you cannot just do generic workouts is because everyone has their strengths and weaknesses. Some workouts are fatigue resistant, some are aerobic power, some are anaerobic. All of these different kinds of workouts have different recovery periods that athletes need to understand and coaches need to stress.
Also certain times of the year are good times to work different body systems. For example, when you hear the words BASE TRAINING. Base training means building a mileage base, but what it also means is PRIMING your body for the competitive phase of training to avoid injury and be strong enough to last an entire season of racing. So there is more to BASE TRAINING besides just running miles and increasing miles. Keep all of these factors in mind while you are training for your next event!