Typically, in events that last "all day" - like Centuries, etc... the easiest method of attaining sufficient fitness is to simply "ride as much as you can" with little respect to intensity.
In most cases this IS the best approach. However, as you have remarked, these rides have significant climbing, so it IS important to stress the muscles you use while climibing.
There's no way to find the "optimum volume" of exercise what you need to do from the little information you supply. However, I can offer a guess about what I would call evidence of being able to finish the rides without "heartbreak".
Try to train up to the level where you can ride 70 miles on hilly terrain that match those rides you want to complete. When you can complete (two) 70 mile hilly rides in the same week without [too much] strain, you'll be more than ready for the "big one"s .......
Otherwise, you need over 200 mile per week for six weeks in a row.