Any runner with big goals—whether it's finishing a first 5K or qualifying for the Boston Marathon—can get worried at the prospect of taking time off from running.
But sometimes it's inevitable: You get the flu, have a really busy week at work, take a vacation with the family, or experience a running injury. No matter the reason for taking some time away from running, the best thing you can do is get back to your normal training volume as quickly and safely as possible.
Even though that's usually the best intention, it doesn't always happen. So how soon do you actually lose the fitness you've worked so hard to gain when you take time off?
More: 7 Tips for Taking Time Off
First, it's important to understand there are two "types" of fitness: your aerobic fitness—in other words, your endurance—and your orthopedic or structural fitness—the ability of your muscles, bones, tendons and ligaments to withstand the impact of running.
Both are important, but both lose fitness at different rates.
How Quickly You Lose Aerobic Fitness
Thankfully, it takes a little while to lose your hard-earned endurance. For most runners, it takes about seven to 14 days for your aerobic fitness to start declining. And what you lose initially is mostly the gains that you've made in the last several months of training.
What exactly does that mean? If you're a lifelong runner, you'll retain much of your aerobic fitness for several months. So don't worry: You won't revert to a couch potato if you need to take a few weeks off due to an injury like IT Band Syndrome.
More: 3 Ways to Conquer IT Band Pain
Here's even better news: The better shape you're in, the more fitness you'll hold onto when you're not running. If you run consistently and have a higher level of fitness than a beginner, you don't have to worry about losing your gains as much as if you were just starting out.
The best use of this information is to run as consistently as possible. Running is truly a long-term endeavor—a lifestyle rather than simply a sport—and your aerobic fitness is something you hold onto for many months.
More: Should You Run More Miles?
How Quickly You Lose Structural Fitness
Structural fitness is critical for injury prevention. It helps you absorb the impact of running without suffering an overuse injury.
This area hasn't been studied as well, but it does take longer to gain structural fitness as opposed to endurance. Many runners experience more rapid declines in this area as well.
More: Hip Strengthening and Mobility Exercises for Runners
This means that for any period of inactivity, your body's ability to tolerate running declines more quickly than its ability to run. This puts you in the injury danger zone.
Any runner will recognize the euphoric feeling you get after five to seven days off from running. You go for your first run and feel great. After all, you've rested for about a week. You may even be running a little faster than usual because it feels better than your usual, slower pace.
But after a week of faster running and feeling good, aches and pains pop up. You may even experience an injury because your structural fitness is much lower than your aerobic fitness.
More: How to Manage Aches and Pains During Training
So what are you to do?
During any period of inactivity, a small amount of strength work can help you maintain your body's ability to withstand running. A short gym workout or a series of medicine ball exercises can often mean the difference between staying healthy and taking even more time off because of a running injury.
Once you understand that your endurance is lost slower than your structural strength, you can modify your training to run more consistently. And the more consistently you run, the more fitness you'll have.
More: How to Become a Consistent Runner and Nail New PRs
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