An endurance athlete who is looking to accomplish big things is in greater danger of working too hard than of not working hard enough. Over the last twenty years as a coach, I have had to hold ambitious athletes back far more often than I have had to push lazy ones forward—even to the point of sending overzealous athletes home when they argued with me that running two minutes slower than 5K pace didn't "feel like running." Most coaches have done the same. When Mary Decker (World and American record holder in numerous running events) began working with Alberto Salazar (the coach of current stars Galen Rupp and Mo Farah) in the mid-1990s, Salazar would prematurely end Decker's workouts if she ran harder than he prescribed.
(Mary Decker set the world record in the mile by running 4:16.71 in 1985. Her coach would end her workouts early if she ran faster than prescribed. Photo courtesy of Bloomberg.)
This is contrary to the "go hard or go home" attitude that many people bring to running, cycling, and triathlon, and it's certainly something that you'll never see on a Gatorade commercial. Nonetheless, anyone who has experienced success in endurance sports will tell you that when they were at their best, their training was a delicately balanced mix between hard and easy efforts. It was only through repetition over time that they were able to reach their goals. As John Parker wrote in Once a Runner, "You don't become a runner by winning a morning workout. The only true way is to marshal the ferocity of your ambition over the course of many days, weeks, months, and (if you could finally come to accept it) years." Parker called this the "Trial of Miles," and indeed, anyone who has been successful in endurance sports has withstood the Trial.
How does going slow make you faster? There are two mechanisms. First: the races that we do are largely aerobic affairs. This means that your body's muscles have all the oxygen they need to produce energy and power. (Of course, they require more oxygen than they do when we are sitting on the couch, which is why our heart rate speeds up and we start breathing hard.) A marathon is over 95% aerobic. A 10K is over 90% aerobic. Even short, speedy events like the mile are up to 80% aerobic. You train the aerobic system by running and riding slowly, not by racing as fast as you can in every workout.
This is not to say that there is no place for going hard and fast. Not only do we need to train that remaining 5% or 10% or 20% that is anaerobic, but there are other things (like neuromuscular coordination and lactate threshold) that we must take into account when we are preparing for our biggest races. Thus, some small percentage of your overall workload will almost always be hard. I often advise athletes who struggle with reining themselves in to hold out for their hard days. In fact, I consider myself an athlete who has to hold out for the hard days. But since the bulk of our races are aerobic, the bulk of our training should be, too.
Going too hard on our easy days not only wrecks our easy days, it wrecks our hard days, too. If a cyclist, triathlete, or runner goes too hard on his easy days or during the easy parts of his workouts, he'll be too tired for the hard stuff, thus making the hard days too easy. Then, because he is annoyed at having underperformed on the hard day, he will go hard on the easy day, thereby sabotaging the next hard day . . . ad infinitum. Ultimately, that athlete ends up doing all of his runs and races at basically the same pace—a pace that is too hard to render maximum benefits to the aerobic system, but too easy to render maximum benefits to the body's other systems. Who wants that? Not your coach, certainly.
Second, and more to the point: athletes who don't take their easy days easy get injured. As many of my athletes have heard me say before, training is essentially strategic damage. When you train, you intentionally hurt yourself so that when your body rebuilds itself, it will rebuild itself stronger than it was before. Running or riding hard stresses your body—particularly your connective tissue—more than going easily. While studies have shown that people who periodically do hard workouts do not have higher rates of injury, similar studies have shown that those who do not let their bodies heal from repeated workouts will ultimately be forced to take time off, either because they are sick, because they are injured, or because they have become "over-trained" (which is essentially an injury to your endocrine system and/or nervous system). If you do not take it easy and let your body heal itself, if you never allow it the chance to rebuild itself stronger than it was before, it will ultimately rebel against you.
(Local runners compete in the Big Peach Sizzler 10K 2014)
So, take heed, driven athletes: slow down. Take it easy. Build your aerobic engine, and allow your body to work its magic. You'll be a faster and stronger athlete in the long run.