I trained for the Mt. Diablo Challenge almost exclusively here on the Valley floor. Similar to botto, CDR, and Merlin: lots of riding at or above threshold. The heart of my training was 2x20' and 5x5' intervals. Into the wind, in a big gear, and on the tops or hoods to simulate the climbing posture.
And dropping ten pounds. For the month prior I cut all junk out of my diet: no baked goods, no soda, no snack foods.