Try to include that 6hr plus strictly Z2 ride if you can. If that means cutting out some intensity then do it. With the long Z2 ride, try and ride till you can feel the legs fatiguing. Then ride a little more and finish.
I’m off on a 5.5 to 6 hour Z2 ride tomorrow. I did one last Wednesday of 5 hours 40 minutes and that’s where I felt the fatigue hitting the legs. When I no longer hit that fatigue I’ll ride even longer.
What I now have is a 50km circuit from home that forms a circle round my home. There are multiple places where I can cut for home. So I go round the circuit, often changing direction after each loop. Then when the legs are fatigued I can cut for home. So it’s easy to extend the time in Z2 without having to commit to more than your legs can currently handle.
In Z2 I’m generally doing a 5-6hr ride, a 3 hour ride, a couple of 1 hour rides. Plus a session of high intensity on the turbo. That’s 5 days a week. Leaving a day for strength work and a day for rest. On rest days I usually go for an easy walk.
My shortest events are generally 9 hours plus. So being fatigue resistant is key to how well they go.