+1 to Mimi. I've never ridden quite that high (I don't think so at least....), but I can tell you that I start feeling the elevation on my bike at around 5,000 feet or so - it feels harder to breather, harder to move - it slows you down. It's not as noticeable if you aren't going all out. In a race it makes me feel like I'm going to hyperventilate and/or puke... on a training ride or a fun ride it just slows me down. Definitely something to think about when planning how much time it will take.

I train for other long hill climbs, like Mt. Baker which is 24.5 miles by doing repeats on a shorter hill - it works fine.