400k is also as long as I want to sit on a bike saddle in one go. Of course one takes breaks every 50 miles or so and I stand every 10 minutes by the clock. Still, it gets old. Not sore, really, just tired of sitting. OTOH, I've had saddles which hurt my butt after only 1 hour and saddles which made my butt bleed. Just the right saddle is the trick, meaning just right for you. Everyone's different. In general for really long rides most folks want enough padding that they can just get their thumb into it. Firm, but some give. Sitting on it should feel like no give, really. Besides that, the shape has to be just right so that it fits your butt shape. It's no accident that Performance bike has hundreds of saddles for sale. There's a lot of variety in butt shape. You're doing enough riding that it's probably the saddle, not just that your butt needs conditioning.
Besides the above, saddles are very sensitive to tilt and height. Frequently just fiddling with those things resolves the issue.
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