This.
Start out slow, get slower, but keep going. Do not push it AT ALL, in the 1st 2 hours especially. Make a point of riding almost comically slowly. If you are breathing heavy, you have overdone it already.
From here to the event, ride as much as possible. Do x hours till your ass hurts. Have a day off. Do x+ 1/2 hrs. Have a day off. Do x + 1 hour, have a day off. If you can ride 4+ hours without too much discomfort, you have cracked it (pun intended).
Germoline creme on your ass before ride, and after if needed. Keep things clean and numbed down there.
Stretch before ride. If getting stiff, stop and stretch.
Drink sports drink, little and often. Light feed every 3 hours. Jelly babies in between.