Time limitations are dictated by children/pick up times etc - if necessary I can set off earlier if factoring in a meal time half way through
Like the idea of maybe 4x 40km “legs” could do a 2x 80km loop with a cafe at the way point
I've just been thinking about this, being in a similar.situation. I'm 67 now and not planning any more Audaxes, but I'd like to ride a few more comfortable Centuries.
I'd say, if you want to ride much farther than you've been recently and you've got a hard time limit, allow enough time to to the extra distance at half the speed (you never know what might happen). . If that doesn't yet allow you to ride 100 miles in nine hours, pick a distance that you could easily do by that metric, see how long it actually takes you, and use that for calculating the length of your next big ride.
So, for example, if you've done 100k in five hours, your next ride should be 140k. If you can manage that in seven hours, you can easily do 160 in nine, while allowing plenty of time for the unforeseen.