As a Whistler regular (10th year in a row this year) the best advice I can give for surviving the long descents is make sure your brakes and suspension are working well. This will save you all sorts of aches and pains.
Set your brakes up so the levers are as close to the bars as possible, without touching when pulled hard, and moved in-board so you can use one-finger braking.
In terms of exercises: I actually found running to be a great help, especially given that you will rarely be seated when on the bike.
Edit re: deadlifts: If you don’t lift weights regularly, don’t start now unless you have professional guidance as you could do yourself an injury.
Similarly with running: Don’t do too much too soon, if not used to it.
How much time do you have?