I did this 6-hour ride in late February, having signed up for an early March double century with a similar amount of climbing, but twice the distance.
I also wanted a good “shock to the body” by doing this ride to force a bump up in fitness. The distance and power levels and feel of the legs post-ride suggest a likely 5-day recovery period which should result in a bump up in fitness. Past experience suggests that to get to the “next level” of fitness requires periodic shocks odf this kind. Double centuries are good for that, which is why I have two scheduled for March.