Training
This is a long hard climb at high altitude, and it's not a
good idea to underestimate how much it is going to take out of
you. You have to be in shape for the approach (long and aerobic)
as well as the climbing. We got ready by putting in a couple of
13-15 pitch days at Lumpy Ridge (e.g., Orange Julius (5.10)/Climb
of the Ancient Mariner (5.10)/Fantasy Ridge (5.9) in a single
day), and agreed afterwards that the Casual Route was still more
tiring.
Climbing Kiener's will let you know how you're feeling at
altitude and let you scope out the approach and descent. Knowing
your way around the mountain will speed up your day on the Casual
Route dramatically.
The Red Wall (5.10) on Chasm View Wall is a great route, and
is excellent preparation for the Diamond; if you can get up Red
Wall with no hassle, you're ready for the Casual Route. This will
also get you familiar with the approach through the Chasm Lake
talus field, which can be tricky in the pre-dawn darkness.
In terms of climbing ability, I was regularly leading 5.11
sport, had done a few 5.11 trad leads, felt pretty solid on most
5.10 trad leads, and had no problems leading any of the pitches
on the Casual Route. When you are ripping up the 10s in the gym
and think you are ready, don't forget that when you get to the
5.10 on the Casual Route you have already
climbed more than 1000', you are 120 feet up the 6th pitch hauling
2 ropes (10 lbs.), with a good-sized rack (another 10 lbs.), at
14,000 ft., and it's probably starting to rain/snow.