It's amazing to me that we still see coaches having their kids run hard workouts on consecutive days and before races. Not that you can never do back to back hard days, but if you do, you'll probably have to follow it up with 2 recovery days. If you're not familiar with this law Read This!
The article shows the result of inadequate recovery time between hard workouts, but you must also apply the correct load of each individual workout. If the load of the workout/race is too high, then the body does not supercompensate. You'll have to take extra time to recover just to get back to where you were before you pushed it too hard. I the load is on the light side, you'll still get some supercompensation and improvement, just not as much as you could have if you'd worked a little harder. So the trick is to apply the optimum load and wait the ideal recovery time (don't forget that easy workouts aid in recovery) before the next "quality" workout. When we're really trying to be the best we can be, we're usually walking a fine line between pushing ourselves hard and over training. It is better to err on the side of undertraining than over training.
We should apply this law when planning both micro and macrocycles in our training; Easy days and hard days, taking recovery week every 2-4 weeks, and breaks between seasons. The only way I know to really nail this is to keep detailed training logs, so that you can look back and make adjustments for the future.
Micro and Macro-Cycles for Base Phase
Recovery times for various workouts