Runners flying into Denver (5,280 feet) for a race often feel it within the first mile — a pace that should be comfortable suddenly isn't. The air isn't "thinner" in a way you can feel on your skin, but there's genuinely less oxygen available per breath, and your body notices immediately even if it takes weeks to actually adapt.
The physiology, briefly
Barometric pressure drops as elevation increases, which means each lungful of air delivers fewer oxygen molecules even though the percentage of oxygen in the atmosphere (about 21%) doesn't change. Below roughly 2,000 feet the effect on performance is negligible for most runners. Above that threshold, unacclimated sea-level runners typically see a real, measurable slowdown in aerobic performance — commonly estimated at around 1% per 1,000 feet of elevation above the 2,000-foot mark, though individual variation here is wide.
Full acclimation takes weeks, not days — arriving 2-3 days before a high-altitude race is often worse than arriving the day before or racing immediately off the plane, because the body is partway through an adaptation process (increased breathing rate, early red blood cell response) that leaves you feeling worse before it makes you better.
Running the numbers
The Altitude Adjustment Calculator takes your sea-level pace and the race elevation and estimates a realistic adjusted goal pace.
Worked example: A sea-level 8:00/mile pace, raced unacclimated at 6,800 feet (roughly Denver's Mile High elevation plus a bit), suggests something on the order of a 5% slowdown from the 1%-per-1,000-feet-above-2,000 rule — closer to 8:24/mile as a realistic target, not 8:00.
Racing it smart
If you can't acclimate for weeks beforehand, the two practical options are: arrive as close to race day as logistics allow (minimizing time spent in the rough middle of the adaptation window), or adjust your goal pace down honestly using a calculation like this one instead of chasing a sea-level number that altitude simply won't let you hit that day.