Do you use a different saddle for a century?

[KnottedYet]

What are the center-to-center and outside-to-outside measurements of your sit bones?

Do you feel you have to scoot to get out of the way of the transition of the saddle from nose to sit area? (Feeling that there is too much saddle in front of your sit bones.)

My first suspicion is always that the saddle is the wrong width or the wrong shape.

Wrong shape (too pear shaped, too much transition, too wide in front of the sit bones) seems to more often cause skin chafing. Wrong width seems more often to cause a deeper pain or ache or bruise sensation. Sometimes only one sit bone gets sore, because the body shifts the other one fully onto the saddle and lets the sore one suffer without support.

Wrong padding causes a whole host of issues, too, but it sounds like you've already experimented with a couple different degrees of padding. The "sitting on concrete" sensation could be from you planting your bones on the outside frame edge of the saddle rather than on the padded "cheeks" of the saddle.

It could be a pretty subtle problem with the saddle, that only becomes obvious with longer rides.