Every runner has heard the advice “quality over quantity.” But what if you can have both? For runners looking to build volume, boost endurance, or target specific race goals, running twice a day can be an effective training strategy to unlock gains in speed, endurance and mental toughness.

The key is knowing how to do it without risking burnout or injury (and understanding that it’s not for every runner). Here’s how to make running twice a day a part of a healthy, sustainable routine that leaves you stronger, not exhausted.

One physiological adaptation: Exercising twice a day may stimulate the production of mitochondria —the part of a cell that plays a role in energy production — according to a 2019 study published in the Federation of American Societies for Experimental Biology. The more mitochondria you have, the more energy you can produce, which translates to improved performance at any speed.

Working out twice a day may also promote improved glycogen storage and energy utilization, says Jay Silva, a certified run coach and certified strength and conditioning specialist. “By the time you get to your second run, your muscles might be a little lower in glycogen, which nudges your body to start burning fat more efficiently — a great adaptation for distance runners,” he says.

Besides the physical payoffs, there’s a significant mental benefit that comes with challenging yourself to get back out there for a second session. “Generally, you’re pretty tired when it comes to that second run,” says Downey. “That ties into the mental challenge of distance running; you need to reframe the way you think about a run when you’re already on fatigued legs, and you have to figure out how to motivate yourself to keep going.”

So how long should you wait between sessions? “Aim to give yourself six to eight hours between sessions,” says Silva. “This break is enough for your body to recover from the first run, so you’re not dragging through the second. Think morning and evening for a natural rhythm.” That also gives you plenty of time to refuel and rehydrate, so you’re not running on empty when you get to round two.

That six- to eight-hour window not only prevents the effects of too much impact from taking you out of the game, but it will also make you stronger in the long run. “Running does cause microdamage to your bones, and it takes them about six to eight hours to go through the rebuilding process,” says Downey. “If you double too close together, you have a higher risk of eventual stress fractures. But if you put enough time between sessions, each one individually signals that the remodelling process will help increase your bone density.”

“A lot of runners like to start the day with a tougher workout, like intervals or a tempo run, then keep the second run easy and relaxed,” says Silva. “That way, you’re getting a solid training stimulus without overdoing it.”

In that case, the lower-intensity run serves as active recovery, helping to remove metabolic waste products generated by the more intense session and increasing blood flow to promote muscle repair. But it doesn’t really matter what order you do them in, says Downey — an easy morning run, for example, could act as a shakeout for a harder evening session — just that one run should be more intense while the other should be lighter and easier.

“If you’re still in the building phase of a training cycle, you can also just do two easy runs,” adds Downey. “A medium or long run in the morning followed by a little more time on feet in the evening can help you accumulate more mileage without overly taxing the body.”

The easy run should be truly easy though — and, following an earlier run, that may feel different than your normal easy pace. For a run to qualify as “active recovery,” aim to keep the intensity to 30 to 60 percent of your maximum heart rate, according to a 2019 review in the Journal of Strength and Conditioning Research. You should feel like you can hold a conversation or like you’re working at a level 4 on a scale ranking effort between 1 and 10 with 10 being an all-out effort — that’s going to prevent you from putting too large of a load on your body.

If you do both runs too hard, you’re digging yourself into a hole you may not be able to recover from. Increasing your mileage too quickly or accumulating more volume than your body can handle can lead to overtraining syndrome, an increased risk of injury, and burnout.

When incorporating doubles, “you’ll probably feel a little sore or tired at first, but if you start noticing signs of overtraining or any sharp aches, don’t be afraid to scale it back,” says Silva.

The key to increasing mileage is to do it very gradually. Runners training for the New York City Marathon who upped their mileage too fast were more likely to get injured than those who gradually added more volume to their schedule, according to 2023 research published in the British Journal of Sports Medicine.

If you’re intrigued by the idea of running twice a day, consider your running history first. Before you can even think about doubling for fitness purposes, you should be able to comfortably manage 65 kilometres a week, plus two strength-training sessions for an extended period of time (we’re talking years, not weeks), says Downey.

Otherwise, “if you’re running under 80 kilometres a week, the only reason to double is if your work or family or other outside responsibilities require you to break a daily run into two sessions.”

If you’re somewhere between 80 and 110 kilometres a week and want to try running twice a day start with just one day a week, “and keep both runs easy,” says Silva. You need to callous your body against that extra stimulus before adding intensity to one of those runs.

Keep the second run of the day duration-based instead of focused on pace, Downey says. “That allows you to listen to your body and keep the effort truly easy,” he explains. It’s smart to keep that run to about 30 minutes or less, too.

Once you get comfortable with one set of doubles per week, you can progress to two doubles per week if needed, but don’t go for more than twice a week unless you’re working with a coach, says Downey.

Also, make sure to prioritise seven to nine hours of sleep, says Silva. “Think of it as helping your body recover and adapt faster, so you can keep up with the double-day rhythm,” he says.

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