The first run at altitude is humbling in a very particular way. Your legs are fine. Your breathing is not. The pace that felt conversational at sea level now requires your full attention, and the climb you would normally run becomes a walk within the first few minutes.
This is not a fitness problem and there is no way to push through it. There is only arriving well.
What is actually happening
Less oxygen per breath means less oxygen delivered per minute, so your heart rate climbs to compensate and your sustainable pace drops. Above roughly 1,500 metres the effect becomes noticeable. By 2,500 it is unmistakable. Expect to lose meaningful time per kilometre at the same effort, and more on anything uphill, where there is no way to back off without walking.
The first three days
Go slower than you think is necessary, then slower again. Run by effort and ignore pace entirely — this is the one situation where the watch is actively misleading. Keep the first couple of runs short and flat, and save anything with real climbing for day four or later. Sleep is usually poor for the first two nights, which compounds everything, so plan less than you want to.
Drink more than you want to
The air is dry and you lose water through breathing at a rate you will not notice. Dehydration at altitude feels exactly like altitude sickness, and it is far more common. Carry water on runs you would normally do without, and keep drinking through the day rather than only around training.
The sun is a separate problem
UV intensity rises roughly ten per cent per thousand metres, and there is usually less shade. A long brim and covered shoulders do more for you here than any amount of sunscreen reapplied badly. Snow or pale rock makes it worse again, because you are being lit from below as well as above.
The reward
Around day five something clicks, and you get a few days of running that feel genuinely effortless when you come back down. It does not last, but it is a good feeling while it does.
High-UV headwear is in Trail Caps and Bucket Hats.
