Your first long endurance event does not need a heroic training plan. It needs consistency, a realistic build-up and enough specific practice that race day does not introduce too many surprises.
Build time before intensity
Start by increasing the amount of easy work you can absorb. Add duration gradually, keep most sessions conversational, and place harder work where it improves the event-specific skills you actually need.
Practise the boring details
Use training to test food, hydration, clothing, lights, navigation, repair tools and pacing. Small problems become large problems after many hours. Rehearsal turns them back into small ones.
Respect recovery
A long session only creates fitness if you recover from it. Sleep, regular meals and easy days are part of training, not time away from training.