As a running addict, I have been forced to learn the hard way time and time again that rest is just as important to fitness gains and improvement as running more miles or adding in more speed work. The thing a lot of us over-ambitious runners seem to neglect is the importance of rest. Sure, it's great to push ourselves sometimes and increase our workload, but we also have to remember that our bodies need time to recover and adapt so we can actually benefit from our efforts. There are a few different ways that runner overdo it:
Training
I found a quote on runnersworld.com that really sunk it, "We train to race, not race to train". This is absolutely right. Time and time again I read more and more evidence proving that constantly pushing your body without adequate rest will only run you into the ground, and more than likely lead to injury and fatigue. None of your workouts should ever cause you to push yourself as much as you do when you are racing. Racing is the only time you should give 100%. Sure, you should include a hard workout once a week, such as a fartlek run, but these runs should still never be executed with as much effort as an actual race.
Along with too many hard-effort workouts, many runners increase their mileage too quickly and don't include cutback weeks. One of the main rules on running is not to increase your mileage more than 10% each week. Increasing your weekly mileage more than that is just asking for trouble. I made the foolish mistake once of increasing my mileage by a little over 20% from one week to another, and ended up with a nasty case of runners knee by the end of the week that caused me to have to take two weeks off. Not giving your body time to adapt to more miles will only lead to injury, so always be sure to follow the 10% rule when it comes to mileage. That being said, this doesn't mean for the next 6 months straight you should increase you mileage by 10% each week. Your body needs breaks and cutback weeks that allow your muscles to fully heal and become stronger. A good rule of thumb is to include a cutback week once every three weeks. A good example of this would be a week with 20 miles, followed by a week of 22 miles, followed by a week of 16 miles. Then, after your cutback week of 16, you could start back at 22, and build, then cutback again. By slowly increasing your mileage and including time for rest, you will reduce your changes for injury and increase your fitness gains.
Racing
Another common issue with overuse is racing. Some runners want to run a race nearly every weekend. However, since racing causes you to really push your body, it's recommended that racing is done sparingly. Plus, the longer distance you are racing, the more time you should take between races. For example, it might be okay to run 3 5K's in one month, but you should never run more than 3-4 marathons a year. While we all enjoy racing and trying to reach new PR's, not taking a break from constantly racing is not only hard on your body, but is hard on you mentally as well. Races take a lot of planning, time, preparation, travel, money, etc. Constantly trying to schedule your life around races is going to drain you mentally and physically. This could possibly lead to feeling burnt out, and could turn something you used to love doing into a stressful chore. Part of the fun of racing is that it's an occasional experience. If you do them all of the time, they start to lose their magic.
Another issue with racing is not allowing your body time to heal afterwards, and the longer the race, the more time you need off. Experts say that it takes your body about a month to fully recover from running a marathon. Now, does that mean you shouldn't run for an entire month? Of course not. However, taking the first 7-10 days after the race off to let your body bounce back is recommended. For most runners this many seem silly, especially if your muscles don't feel very sore afterwards. However, running 26.2 miles is very hard on the body, and it takes a toll on a lot more than you think. After a marathon, the immune system is compromised, muscles are torn, and cells are damaged. (Yes, that's right, the cells in your body are damaged!) So even if you have the energy to run a 10K the following week, you still shouldn't. Many professional and Olympic marathoners take up to two whole weeks off after a race to fully recover before easing back into running. Taking some time off isn't going to cause you to lose all of your fitness or set you back a bunch. Taking a whole week off on running will only cause the average runner to lose 3% of their overall VO2 max, which can easily be gained back in a week or two of regular running.
Injury
Lastly, when it comes to injury, most runners are pretty stubborn. I know, I'm one of them. When an injury happens during training, it can be difficult to allow yourself to take time off and fall behind. I have foolishly ran through injuries before in order to keep training, only to end up making it worse and eventually taking even more time off. It's always better to opt to take a day or two off of training than to try and keep running on it, eventually forcing you to take weeks off. I've learned that it's always better to be safe than end up making matters worse. listen to your body, and know when to say when.
Running can,(and should)be a lifelong activity. As long as you approach it the right way, it will do more good for your body than bad. As long as you let your body rest, heal, and adapt accordingly, you can build a strong base for running throughout you lifetime. So next time you overdo it, or are forced to take some time off, don't look at it as getting behind on your fitness, or falling behind on your training; think of it as helping your body build a stronger athletic base that will benefit you in the future!
