I was thinking the same thing as AB and Alpinerabbit - you sit bones are what you want in contact with the saddle, so pressure on them is normal and even desirable - but severe pain after 20 min doesn't sound right either.... Butt break in usually feels more like really tender when you first sit back on the saddle the day after a long ride then goes away after a few minutes back on the bike. (And eventually stops happening altogether)

I'm wondering if the saddle is just a little too narrow and your sit bones, while still in contact with the saddle and protecting your soft tissue, are trying to slide off of the edges?