I've done a few long multi day rides on my XChallenge (stock seat, aka 200mm plank) - 1800km in 3 days, with some 640km days here and there
Off road/gravel it is totally fine, but long stretches on tar at 120km/h +, at the end of a 3 day ride seriously chows my butt.
On day 3 of a previous trip...