Whether you're gunning for a 5K or a marathon, there's no doubt that recovery is an important part of training. The trap many runners fall into, however, is allowing too much or too little time for recovery.
Just because you did a long 20-mile run, doesn't mean you should put your feet up for the next three days. You'll be hard pressed to build a good fitness base with that approach.
Conversely, putting in one hard run after the next with little to no rest will negate the desired training effects in a different way. In this case, you may end overtrained or injured, which you could have potentially prevented with adequate rest.
The best approach to recovery, "active recovery," falls somewhere between these scenarios.
While there are certainly instances where your body needs rest, much of the time, an easy run can actually help speed up the recovery process after a hard effort by increasing blood flow in the legs. Not only can this help flush the lactic acid from your muscles, it can also cut down on some of the muscle soreness after a long or hard workout.
"Runners improve performance by training at different paces—long, slow distance runs, speed workouts, 'sneaky speedwork' like tempo runs and fartleks and easy recovery runs," explains USATF and RRCA-certified running coach Brendan Cournane. "In order to build on the conditioning between hard workouts, use easy runs or cross-training to maintain aerobic fitness and conditioning."