Starting now? Get in good shape - ride a lot.![]()
Altitude affects everyone differently. If you have any opportunity to go ride at altitude, even for a day or two, it would be great to do that so that you know how you are affected.
When we train sea level-living people to ride the Death Ride (6500 - 8800 feet), we make them (1) ride their butts off and get in shape so they're ready to climb the passes and (2) spend a weekend riding at elevation. Doing this helps them understand how the elevation will affect them. Many folks find their stomach is affected and that they have trouble taking in adequate food and staying hydrated. Thinner air is often drier, so hydration is hugely important. If you have a chance to ride at higher elevation, you find out what works for you in terms of what your stomach can tolerate.
In terms of thinner air and breathing - this is where the training and getting in as good shape as possible comes in. You'll generally go a bit slower, breathe a bit more. People with asthma may need to monitor themselves more closely. But as long as your pacing yourself, you're not going to be gasping for air. (I'm assuming the elevations you are talking about are roads below about 9000 feet. I do notice some gasping for air in the higher mountain trekking I have done - over 10 - 12,000 feet it really starts having an effect on some folks).
As for acclimitization - you are not going to acclimate to elevation with much less than a week to 10 days at altitude. They've done studies that show that spending only a few days at high elevation actually decreases your performance. So unless you can go up at least a week before your event, don't sweat it. Get there the day or so before and just relax - and drink a LOT of water!!!
Alternatively, you could get one of those fancy altitde tents and sleep in it! But I don't think that's what you had in mind...![]()