I've really been enjoying the multitude of Olympic sports broadcast over the last 2 weeks, but the mountain biking really was exciting to me since I pedal my fat tire bike over 2500 miles each year, though admittedly never on a course like they had in London. They race 18.2 miles -- 6 laps of the 3 mile course -- so the obstacles are repeated 6 times. And the fastest women do each lap in 15 minutes, an astonishing 12 mph average. And each lap has 500+ feet of elevation gain, too!
Here are some photos I found online to give you an idea of the course (click to enlarge):