You don't really need mega rides.
They really just make you overly tired.
You're better off doing shorter rides and going harder.
I rarely do any rides over 4 hours.

That being said, I love the odd mega ride.
I have an insanely hard 170 km planned in a couple of weeks, and a 200 km ride in about 8 weeks.
Both will take somewhere in the 8 hour range.
I'm low 5 hour for IM.
My A race is about 20 weeks away.