I'm in agreement with the rest - jumping from a daily mileage of 20 to 100 is not really feasible. You can do 20-mile days for 100 days in a row and all this training will help you do 20-miles more comfortably, but it won't prepare you to do a 100-mile day or even a 50-mile day.

My advice is that you continue increasing your training mileage to the point where you can comfortably complete 50 or preferably 75 miles. With this, you'll probably be able to push yourself on the big day to do the 100 without being dangerously over your head. It may not be pleasant but you can probably do it.

- Mark