P68 will cruise at 150 kts, faster still if you pour more fuel in to it.
It will loiter comfortably at 80-100 kts burning a lot less fuel.
As for physiological endurance, I used to regularly fly 8 hour surveys (now flying kerosine burner that can't dream of staying up that long). I've never used a travel john... but the advantage of a fixed wing is "look Mum, no hands!"
