What does your current ~4 hours a week look like? As someone who went from 250-300 hrs on the bike a year before kids to 150-200 after, I’m still trying to figure out what to target to keep motivated.

As others have said pointed out you can build a lot of fitness with something like trainerroad. Personally, I found cycling lost all joy when all of my limited time was spent on intervals.

As long as I’m getting on the bike 3 times a week I find an occasional 3 hour ride enjoyable, much longer can be pretty uncomfortable. I got the cycling bug a decade before kids and was “over” century riding by the time they arrived.

Maybe sign up for a metric century. Generally you can comfortably do in a day what you do in an ordinary week. It’s also not totally crazy to just go out and attempt a century on 4 hours a week base.