This is a classic thread and something I repeat often, try learning to ride on feel, not enough people do it or can manage it.
Use the tools you have now, power meter, buy a HR monitor and understand what it feels like to ride at various intensities whilst looking at the numbers. Trainer workouts will feel harder than road sessions, but keep an eye on the numbers and try to remember what it feels like. Perform this over the many months leading up to the event and you should be fine.
Remember, nutrition plays a huge part in long distance rides. It’s easy to say sit at x watts or x BPM but without the correct nutrition those numbers will be pointless as they won’t be attainable.