You might need something a little more specific for the weeks leading up to your event. The closes thing we may have is our gravel plans. Gravel races are long aerobic event for the most part which is exactly what you will be doing. You won’t be riding at threshold or anywhere near, but need to build a huge aerobic engine. So doing 18 weeks of sweet spot is a great start!
Why I say gravel plan is that these offer the most aerobic work but also on the weekends there are simulation rides. This is what you will need to do and they are long rides that the plan is built around. Slowly increasing the duration of your rides so you are prepared to do a 24 hour ride. Working on your hydration, nutrition plan, riding the same equipment and knowing how comfortable your position will be. You wouldn’t actually do a 24 hour beforehand but it would be good to do at least a 12 hour ride 3 - 4 weeks before hand.