Ever take a brief break from running, and feel like you're instantly starting back at square one? Here's why, plus how to return to your regular badass runner self ASAP.

By By K. Aleisha Fetters
June 19, 2017

You ran a marathon a month ago, and suddenly you can't run 5 miles. Or you took a couple weeks off of your regular SoulCycle sessions, and now making through a 50-minute class is hard as hell.

It's in no way fair, but it's how good ol' biology works. After all, in all things fitness, you're either training or detraining. That seems to be particularly true when it comes to cardio.

"The benefits of cardiovascular training are more transient than those in strength training, meaning they occur quickly and go away quickly too," explains Mark Barroso, C.P.T., a New Jersey-based trainer and Spartan SGX coach. "Once cardiovascular training is stopped for two to four weeks, you may see reductions in your respiratory ability, VO2 max [the maximum amount of oxygen your body can take in and use in a minute], and your body will become fatigued more easily."

What gives? It all comes down to the biological changes that occur in your body when you're performing your workout of choice. "With endurance training, we don't need to dramatically change the structure of our bodies to be able to do it," Barroso says. (FYI, with strength training, it generally requires taking at least six weeks off of your workouts to see decreases in muscle, tendon, and ligament size and strength.) "We only need to teach our bodies to deliver and efficiently use the oxygen and substrates and transport waste products," he says. Those responsibilities largely fall to metabolic enzymes and hormones, which are highly responsive to aerobic exercise-or a lack thereof.

In fact, Chris Jordan, C.S.C.S., C.P.T., director of exercise physiology at the Johnson & Johnson Human Performance Institute, notes that within as little as two weeks, the activity of oxygen-processing enzymes in the body's muscles decreases and the muscles begin to hold less and less glycogen, your body's stored form of carbohydrates. There's a decrease in the number and concentration of blood capillaries in your muscles, which help deliver oxygen to your muscles and clear out waste products such as hydrogen ions, he says.

Take one Nutrition, Metabolism and Cardiovascular Diseases study. Adults stuck to regular cardio routines for four months straight, and then took a whole month off. They lost almost all of their aerobic gains. Their improvements in insulin sensitivity and levels of HDL (good) cholesterol also all but disappeared.

If you want to look on the bright side, though, they didn't gain back the belly fat that they had lost while training. And their blood pressure levels stayed in check.

So is there any real way to keep your cardio up while taking a break from your regular heart-pounding workouts? (That vacation isn't going to take itself, ya know.)

Jordan says that maintaining cardio fitness requires a minimum of three days per week of vigorous training. (Muscular power and strength can be maintained with as little as one day per week.) That's probably more than you were hoping, but it's also significantly less time than you spent training for that half marathon. (Consider one of the best cities for runners for your next vacation.)

In the end, though, life happens and you're going to need an extended break at one point or another-that's okay. The most important thing is to not let the frustration of "starting over" keep you from jumping back into your routine. After all, while it can take anywhere from weeks to months to build your cardio back up, it will undoubtedly take less work than it did the first time around, Jordan says.

Now get out there and run.