I've done a couple of 200s, one 300, and a 400. Average speed goes down the longer you go, due not only to slower pacing but just more shit going on. I finished a 200k rando event in under 8 hours, did 220 in 10 hours, 300 in 15, and the 400 took 19.
All of these rides were accomplished as a team and not solo. The longer rides were done with a trusted riding partner. Someone you actually like spending time with is key. This is like road tripping with mid day bonks, flats, and so on.
Shitloads of bananas, coke half cut with water, clif bars. Water, water, water. Extra tubes, patches. And a proper bike fit if you haven't done that already.
If you really want a "respectable" finish but haven't done this kind of distance before, I would highly recommend doing some shorter rando events to practice.
And, always stop for a photo when you reach a summit ;)