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“The worst thing you can do is to go up on the day of competition [or a hard training run] without allowing your body any time to adjust to the elevation gain,” says Hackett. “That has been proven to ensure you will lose the race.” Ideally, athletes should spend seven to 14 days at a particular elevation before competing, but Hackett says a three to five day acclimation period will also help improve performance, as most athletes will have overcome any symptoms of AMS at that point and their ventilation systems will have acclimated to the heights.

When booking your travels, consider an overnight stay at an intermediate altitude—say, stopping in Denver at 5,280 feet before ascending to Vail at 8,120 feet. This can be “very helpful” with the adjustment process, says Hackett.

“I’m a big advocate for going on an easy, 30-minute run as soon as possible once you arrive,” advises Megan Lizotte, a professional distance runner and running coach with Hit the Ground Running. Lizotte currently lives in San Diego but returns to her hometown of Aspen, Colorado for about four weeks every summer, where she works with many sea level clients in town on vacation. “This initial shakeout run allows you to get a taste of what you’re in for and re-energizes you from the travel.”

One of the first things you’ll notice at high altitude is the need to breathe faster and deeper, as your body needs to take in more oxygen than usual, explains Hackett. This can result in a general shortness of breath feeling that can last the first two to three days and is (not surprisingly) amplified with exercise.

When Lizotte sees a sea-level client gasping for air during an initial high altitude run, she encourages them to first relax their jaw “because that calms down a lot of other muscles connected to the rest of your body.” Then, she’ll have him/her breathe out with an “ssss” or “th” sound, which helps open the vocal chords. This technique is especially effective during hill repeats or other high-intensity workouts that generate lactic acid quickly, as it “takes the focus off of the pain and sense of panic that runners can feel,” says Lizotte.

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Your body naturally dehydrates at high altitude, explains Hackett, stressing the importance of monitoring hydration levels and adjusting water intake accordingly.

On the flip side, he’s seen runners drink too much water, which dilutes sodium levels and can increase risk of hyponatremia, a condition that’s easily confused with altitude sickness, and can be fatal if untreated. “In reality you only need an additional liter to a liter and a half of water per day at altitude,” says Hackett. Two good rules of thumb to help you strike a healthy balance: first, weigh yourself before and after a run so that you have an accurate understanding of just how much fluid you need to replace, and second, check your pee. Clear urine means adequate hydration while a dark stream spells dehydration.

Related: The Weird Symptoms of Overdrinking During Your Race

VO₂ max—the maximum amount of oxygen that an individual can use during exercise and a metric long used as a measure of running fitness—is markedly diminished at altitude, explains Hackett. This means that if you try to exercise at the same intensity at high altitude that you do at sea level, you are at a much higher percentage of your maximum effort. Starting at around 5,000 feet (around the altitude of Albuquerque), VO₂ max drops 3 percent per thousand feet of altitude gain.

“Athletes who don’t properly pace themselves at high altitude get exhausted and collapse,” warns Hackett, who has seen “a lot of comatose marathoners at 12,000 feet because they don’t understand the acclimatization process.”

Related: Tips to Beat High Altitude Sickness₂ Max?

His advice: take the first couple days at altitude especially easy. Understand—and accept—the fact that the altitude decreases your aerobic abilities and dial back your training plan and/or race goals accordingly. SImply put: “It is impossible to run a mile at high altitude as fast as at low altitude.”