Heading out for two runs in a single day—logging “two-a-days”—is standard practice among elites. But most of us wouldn’t dream of running twice a day—either we don’t have enough time or there’s too much injury risk. However, it may be time to rethink your own training strategy.

“There is a misconception that doubles are something only high-mileage, elite runners do,” says Steve Magness, an exercise physiologist and cross-country coach at the University of Houston. “But a lot of runners can benefit from them.”

For good reason: Studies suggest doubling up and running in a depleted state can boost fat-burning, train the body to use glycogen more efficiently, and stimulate mitochondria production (more mitochondria can delay fatigue).

“By shortening the time between runs, you’re challenging your body to recover faster,” says Greg McMillan, an exercise physiologist and founder of McMillan running. “And a faster recovery is a good thing.” But bad things can happen if you overdo it. Here’s when and why running twice a day makes sense—and how to do it safely.

When to Run Twice a Day

“Cumulative mileage matters—no matter how you do it,” says Brad Hudson, author of . You can boost your total miles by doubling once a week—and still keep a rest day. Four to 10 hours after a key workout such as an interval session or a tempo run, go for an easy 20- to 45-minute run, and don’t fret about pace. This will boost mileage and aid recovery from the first workout by increasing blood flow to the muscles and flushing out lactic acid and other metabolic waste products.

The result? Fresher legs for your next run. Sometimes, “the best massage you can get is from a second run,” says Hudson. On days you can’t bear the thought of lacing up again, try pool-running, cycling, or the elliptical. Such options offer similar recovery benefits without the pounding, says Hudson.

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Why You Should Try Running Twice a Day

It can be tough to run six to eight miles on a Wednesday, no doubt. So divide the run in two, and you can reap a surprising number of benefits. For example, logging two 40-minute runs delivers a double boost of human growth hormone (production peaks about 40 minutes into a run), which helps build and repair muscle. You’ll also enjoy two post-exercise spikes in your resting metabolic rate, which could aid in weight loss.

And finally, you can push the pace a bit on the shorter runs. “Sometimes it’s better to take two runs that you feel really good about than one that you just slog through,” says Hudson. That said, there’s simply no substitute for the weekly long run when it comes to building endurance, muscle strength, and mental readiness, particularly if you have a half or full marathon in sight. Maintain your long run and key workouts and split only mid-length recovery runs, says Magness.

How to Use Two-a-Days for a PR

Seasoned, experienced runners with a solid aerobic base and a lofty time goal can increase strength and speed with a regimen of two daily targeted workouts. “This approach is for people who have pushed their training as far as they can and are looking for an extra boost,” says Magness. To your quality days, add a second, shorter intense workout (one-half to three-quarters the length of your first run) tailored to your specific race goal. So if you’re trying to improve speed over distance for a half or full marathon, follow a morning tempo run with an afternoon endurance-building session of, say, 3 x 1-mile repeats at slightly faster than tempo pace.

If you’re targeting a PR in a 5K or 10K, do your usual speedwork in the morning and add shorter, faster hill repeats or intervals in the evening, says Hudson. For example, 4 x 2-kilometer repeats at 10K pace with 3 minutes recovery followed by 4 x 400 meters at 3K pace with 1-minute recovery.

“Most workouts, you come in pretty fresh,” says Magness. “With these blocks, you’re coming in tired. You have to use muscle fibers you don’t typically use, so your body has to develop a different strategy to dig a little deeper.”

Such efforts make your warmup, cooldown, and recovery even more critical. On rest days, really rest: Take the day completely off. “It can actually make people smarter about their training to know these double days are coming up,” says Hudson.

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