If we look at existing high altitude adaptations in humans we can probably get a clue as to what they might be like, should they survive long enough to reproduce.
Adaptations for low oxygen
By far the biggest selective pressure on your people will be hypoxia during pregnancy, which is a big problem. On the positive side, issues affecting fertility tend to get selected out very quickly.
Tibetans show the most adaptation to high altitude of all the high altitude populations. They breathe more rapidly than lowlanders, inhale more air with each breath (larger lungs), and have increased levels of nitric oxide in the blood (allows for greater dilation of blood vessels). Interestingly, they don't show elevated haemoglobin levels (a common adaptation). Their normal haemoglobin levels is thought to protect them from chronic mountain sickness.
Andeans and Ethiopians display other adaptations, notably increased oxygen-carrying capacity of their haemoglobin.
I suspect that your high-altitude people will need to use all of these adaptations, and increased in efficiency where possible. They may well have reduced capacity for exercise in their homeland, although it would be far greater than a visiting lowlander.
Another interesting adaptation they may use is a sort of blood-doping used by horses. Horses store fully one third of their red blood cells in their spleens at rest. When they need rapid bursts of speed, this blood is shunted into the rest of their system and produces a burst of oxygen in excess of what their lungs can supply. If your highlanders develop a similar adaptation then they may still be capable of bursts of speed and strength when they're otherwise sluggish.
These are all relatively easy adaptations as far as things go. They're tweaks to existing systems, rather than whole new ones.
Adaptations to cold
Luckily, this one is pretty straight forwards. Stockier build, thicker fat layers and more hirsute are easy adaptations. I'd expect them to have larger noses as well (the nose acts as a heat exchanger to warm incoming air before it reaches the lungs).
I'm not certain if the timeframe is long enough, but if they can develop translucent hair and dark skin like polar bears then that would help retain heat from the sun as well.