The Pros and Cons of Running Twice a Day

By Lisa Marshall • Runner's World

Heading out for two runs in a single day—logging "doubles" or "two-a-days"—is standard practice among elites. But most mortals wouldn't dream of it: not enough time and too much injury risk.

"There is a misconception that doubles are something only high-mileage, elite runners do," says Steve Magness, an exercise physiologist and cross-country coach for the University of Houston. "But a lot of runners can benefit from them." Including time-crunched folks trying to squeeze in miles and veterans looking to step things up. 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).

More: Double Up Your Daily Runs

"By shortening the time between runs, you're challenging your body to recover faster," says Greg McMillan, an exercise physiologist and coach in Flagstaff, Arizona. "And a faster recovery is a good thing." But bad things can happen if you overdo it. Here's when it makes sense to double up—and how to do it safely.

How to Plan for Back-to-Back Marathons

Little Time, Big Dreams

"Cumulative mileage matters—no matter how you do it," says Brad Hudson of Hudson Elite Marathon Performance in Boulder, Colorado. You can boost your total miles by doubling once a week—and still keep a rest day.

4 to 10 hours after a key workout like 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.

"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 (try these four workouts borrowed from other sports to mix it up.) Such options offer similar recovery benefits without the pounding, says Hudson.

More: Distance Running: How Many Miles Should you Run?

Mid-week Mileage Crunch

No doubt, it can be tough to run 6 to 8 miles on a Wednesday. 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.

More: 3 Reasons to Include Recovery Runs in Your Training

"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.

Which Matters More—Speedwork or Mileage?"

Aiming 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 (one-half to three-quarters the length of your first run) intense workout 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 a.m. and add shorter, faster hill repeats or intervals in the p.m., says Hudson. For example, 4 x 2-kilometer repeats at 10K pace with three minutes recovery followed by 4 x 400 meters at 3K pace with one-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 warm-up, cool-down, 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.