Just pace yourself, drink and eat often. You will be fine.

I've done the Mt Evans climb in Colorado. Starts at 8000ft, ends at a little over 14,000. I was starting to get pretty lightheaded at 12,000 and had to throttle way back at 13,000. Even at 6-7MPH, I was still getting slight tunnel-vision. But it never worried me and I felt OK when I hit the summit. What a great ride. If you push yourself too hard, it will definitely push back!