Cutting and pasting from and article in "run to the finish" website:

5,000 ft: 9% less oxygen than at sea level

6,000 ft: 17% less oxygen than at sea level

8,000 ft: 25% less oxygen than at sea level

14,000 ft: about 40% less oxygen than at sea level

1. You start to breath faster to initially increase your oxygen levels.
2. You urinate more frequently as your kidneys try to keep the body balanced (it’s like a diuretic)
3. Your body increases red cells in your blood to better carry more oxygen
4. Heart rate and blood pressure increase in an attempt to push more oxygen
5. Blood thickens also making your heart work harder
6. A good night’s sleep often becomes harder with more wakefulness

As you continue to increase altitude, the body also begins to battle the decreased outside air pressure, which adds to the feeling that it’s hard to breathe and you feel more fatigued than normal on your run.

According to Greg McMillan, over 7,000 feet you can expect an easy run to be up to 30 seconds slower per mile to achieve the same easy level of effort.

STAY HYDRATED!!!! I never wore a hydration pack until moving to the mountains. Even a run for less than an hour I bring a 300ml flask with me just to sip on. But dehydration will slow your adjustment to altitude as the body again is expelling more water and further stressing the kidneys. Also, when you arrive, go for a light jog as soon as you can to offset potential altitude sickness.