You ran a comfortable 8:30 pace at sea level for years. You moved to Colorado in August, went out for an easy four miles that first week, and spent most of it wondering whether you had gotten badly out of shape in the three weeks it took to pack a moving truck.
You did not. Denver sits roughly a mile above sea level, and Wheat Ridge and the western suburbs sit a bit higher still. That changes the math of running in ways that are predictable, mostly temporary, and worth understanding before you decide something is wrong with you.
What altitude actually changes
The percentage of oxygen in the air does not change as you go up. What changes is barometric pressure, which means each breath delivers oxygen at a lower pressure, and less of it crosses into your blood. Your body compensates in a handful of ways:
- You breathe more. Both rate and depth go up, at rest and especially during effort. This is why your breathing feels out of proportion to your pace.
- Your heart rate runs higher for the same pace. A heart rate that used to mean tempo effort may now show up on an easy jog.
- You lose more fluid than you expect. The air is dry and you are breathing more of it, so respiratory water loss goes up alongside sweat.
- Sleep is often disrupted early on. Many people report broken sleep in the first nights at a new altitude, which does the rest of your training no favors.
Over weeks, your body makes longer-term adjustments, including changes in blood oxygen-carrying capacity and in how efficiently muscle uses what it gets. This varies a great deal between individuals, which is the part most training articles skip.
How long adjusting takes
There is no single number, and anyone who gives you one precisely is overselling it. The general pattern most runners describe looks like this:
- The first week or two are usually the roughest. Easy runs feel moderately hard, hard runs feel unreasonable, and recovery is slower.
- Weeks three through six are when most people notice real improvement, particularly at easy and moderate efforts.
- Several months in, many runners feel close to normal for everyday training, though top-end speed often stays somewhat below sea-level numbers.
That last point is worth sitting with. Adjusting to altitude is not the same as erasing it. At a given altitude there is generally some reduction in maximal aerobic capacity that does not fully go away, and it tends to show up most in longer, harder efforts. Plenty of strong runners live and train here. They just run their race paces at the paces this air allows.
How to train through the transition
Run by effort, not by pace
This is the single most useful change. Your old pace chart was built at a different oxygen pressure. For the first several weeks, run easy days by breathing and conversational effort and let the watch report whatever it reports.
Delay the hard stuff
Hard intervals in week one are how people end up both discouraged and injured. Most runners do better keeping volume modest and intensity low at first, then reintroducing quality work once easy running stops feeling like a workout.
Take hydration and fueling seriously
Dry air plus higher breathing volume plus Colorado sun adds up. Carry water on runs you used to do dry, and pay attention to salt on long efforts. If you are reliably getting headaches on afternoon runs, under-hydration is a reasonable first suspect.
Respect the terrain separately from the altitude
Many people who move here add trails and hills at the same time they add elevation. Those are two different stresses on your legs. Climbing and descending load tendons and joints differently than flat pavement does, and the Achilles, knees and shins are usually the first to complain. If something starts hurting, our sports medicine page covers the injuries we see most often from exactly this transition.
Expect sun exposure to be different too
Higher elevation means less atmosphere filtering ultraviolet light, and Colorado has a lot of clear days. Sunscreen on long runs is not fussiness here.
What is normal adjustment and what is not
Feeling winded, slower and more tired for a few weeks is ordinary. Some things are not, and they are worth distinguishing.
Worth a conversation with a physician:
- Shortness of breath that is out of proportion to your effort, or that shows up at rest
- Chest pressure, tightness or pain with exercise
- Lightheadedness, near-fainting or fainting during or right after a run
- A heart rate that stays high long after you stop, or an unusually irregular pulse
- Wheezing or a persistent dry cough with exercise, especially in cold dry air
- Fatigue that keeps getting worse after a couple of months rather than better
Some of these are about altitude and some are not. Exercise-induced bronchospasm, iron deficiency, thyroid problems, sleep-disordered breathing and cardiac issues can all look like a bad adjustment to elevation, and sorting that out usually takes a history, an exam and sometimes labs. Appropriate evaluation is worth asking for rather than assuming the air is the whole story.
Go to urgent care or the emergency department for chest pain, severe or sudden shortness of breath, confusion, a severe headache with vomiting, coughing up pink or frothy sputum, fainting, or any symptom that feels frightening rather than merely uncomfortable. The more serious forms of altitude illness are uncommon at Denver elevation but are far more relevant if you head up to a fourteener or a mountain town, and they are emergencies.
The things people wish they had done differently
- Keeping their old training plan intact. The plan was written for different air. Adjust it, do not grind through it.
- Racing in the first month. If you can move the date, move it.
- Comparing themselves to locals. Many of the people you are running past have been breathing this air for years.
- Ignoring a new ache because they assumed everything was altitude. Shortness of breath is altitude. A sharp pain in your shin is not.
If you are not sure whether what you are feeling needs an appointment, our post on when to see a sports medicine doctor is a reasonable place to start.
Where a physician fits into this
Most runners who move here need information and patience rather than treatment. Some need a real workup, and some have a brand new training environment layered on top of asthma, high blood pressure, anemia or a heart condition that deserves a second look before they push hard.
Dr. Christopher Kroner is a family practice and sports medicine physician who has worked with athletes at the professional and Olympic level as well as on high school sidelines, and he has spent years looking after people training in this exact air. Because visits here are unhurried, there is actually time to talk through your training log, your symptoms and your history in one sitting rather than across three appointments. Primary care and sports medicine sit under the same roof, which helps when the question is whether the problem is your lungs, your heart, your iron or your plan.
A direct primary care membership is not health insurance. It covers your access to the practice, and most patients carry a separate insurance plan alongside it for hospital care, imaging and specialists.
If you are new to the Front Range and want someone to look at the whole picture, you can book a free 30-minute meet and greet with Dr. Kroner or call 720-807-9961. The office is at 4045 Wadsworth Blvd #300, Wheat Ridge, CO 80033, open Monday through Friday, and we see patients from Arvada, Lakewood, Golden, Edgewater and west Denver.
This article is for educational purposes only and is not a substitute for individual medical advice. Please see our medical disclaimer.