I'm not expert either my longest ride is 139 miles. But I do think there are a lot of issues that come up later in rides that you don't have to deal with on shorter ones. (I'm talking at 110 versus 60 miles).

But one suggestion is to do back to back days of really long rides. I'm just making this up based on my own experiences, so take it with a grain of salt.

But could you first do back to back days of centuries on fairly flat roads, to get the feeling of being in the saddle longer? You coudl start early and then have the later afternoon and evening with your family.

Then maybe a month later try to do the back to back centuries on hillier roads?