Elevation is definately something to be concerned about. I did Mt Whitney about 10 years ago with my brother. At the time I lived at about 300ft elevation. We camped at Whitney portal at 8900ft for 3 days to aclimatize, and then did the main trail which is a 22mi round trip hike (no real climbing) over 3 days. Even with 3 days to get used to the higher elevation, the hike nearly did me in. Torwards the end I would take a few steps, stop and breath, take a few more. Because my body was used to the almost sea level elevation, I was significantly affected by the elevation.
While the atmosphere is always 21% oxygen, the lower pressure at higher elevations means there is less molecules for a given volume, and thus less oxygen for each lung full of air. The below numbers give you an idea of the equivilant persentage if at sea level.
Sea level is 21%
8900 Whitney portal is 14.8% equivilant
11000 Trail camp was 13.7% equivilant
14505 aprox summit was 12% equivilant
20322 McKinley is about 9.7% equivilant
So near the summit of Whitney there was just over half the available oxygen I was used to!
I really would not throw yourself at McKinley with no guide and little preparation, living and hiking in the 1500-3000 ft range.
Elevation affects different people in different ways. If you want to see how it affects you, Whitney is a great way to try yourself out. The trailhead is easily accessable by car. The main trail is essentially a 22mi round trip hike, no mountaineering equipment is required, and water is available along the trail(bring a filter) It can be done in a day (my more athletic brother did it) if you are in good shape and day permits are not as hard to get. You will know pretty quickly if elevation is going to bother you a little or a lot!
Good luck!