Once a week, couple months before, start adding weight to the bike, about 5 pounds a week, then do a hilly ride.
Doing long training rides is a good idea, but you don't need to double the distance. If you want to do that, fine.
Btw, you could always use a periodised training program. These can be complicated to set up. I'd use a coach or coaching service. But you can find rough
descriptions of how to do it in some of Friel's books.