it's probably too late for some of this advice since you are already there, but for what it's worth and for anyone else contemplating riding at altitude:

I recently did a ride in Colorado with basically no problems:
The highest point was a little over 11,000 feet

- I began drinking lots of water about 3-4 days before the ride
- I arrived at ~5000 two days before the start of the ride
- I started taking a medication called Acetazolamide about 24 hours before the ride (mostly worried about my asthma)
- I continued drinking lots of water throughout the ride!!!

not recommended but in case you were wondering:
- I had 1 beer a night (OK, two on the day off )
- I had 1 cup coffee each morning

The only issue I recall (and this may have nothing to do with the altitude) was feeling a little dizzy the first night. I laid my head down (late afternoon) and started feeling a little dizzy. I immediately felt better after getting up and walking around and never had another problem after that....