You know the expression "What doesn't kill you makes you stronger"? When it comes to fitness, it's totally true.

An example: A few years ago, I spent 11 days hammering through the French Alps with several friends. No ride was shorter than three and a half hours. I'd never ridden so hard. But on my first ride back home, I could barely pedal. I told myself I was "overtrained" and put the bike away.

Less than a week later, I went on a local group ride to see if I could hang. The peloton was mostly pros and other categorized racers, and it usually took everything I had just to cling to the back. But on that ride, I was the guy pulling off the front. Which made me wonder: Was I ever overtrained?

"Overreached is the better term," says Allen Lim, coach and physiologist with the Garmin-Chipotle pro cycling team. "Overtrained means you're really screwed up. Overreaching is something you strive for, because if you rest a person who's overreached, he comes back stronger."

Unfortunately, we part-time athletes don't get that message. Whether building explosiveness for skiing or endurance and power for running, we live in fear of overtraining and fail to push hard enough.

And while overtraining is a serious condition affecting a tiny percentage of athletes, Lim has seen only a handful who fit the description in 12 years as a pro coach.

"Even if you're doing two-a-days for three weeks, you're not really at risk of overtraining," says Dr. Steven J. Keteyian, a distance runner and the program director for Preventive Cardiology at Detroit's Henry Ford Hospital. "Take the weekend or a week off and you'll be better."

The takeaway: When the time is right, go harder, then rest. "The hardest part of being an athlete is forcing yourself to rest," says Henderson. "Overreaching breaks you down until you feel it's worth resting."

But coordinating overload, recovery, peaks, and goals is tricky. To do that, you need to get on a periodization program.