I don't have time for long rides except when my grown son-in-laws visit so several times a year we will go for a few back to back 50-90 mi rides.
I ride as hard as I can for 25 mi 5 days a week (before work). So I'm pretty comfortable to about 35-40 miles. At 30 mi I change positions more and make sure I've got my backstroke working. By the end of the ride I'm spent but so is my son-in-law and he's younger, stronger and rides more. At 63 endurance, not physical prime, is on my side. At 28 even though you are at physical prime endurance is still developing.
Ride hard as much as you can. Even when pulling your girl (lucky you!) pull some hills. Riding hard every day goes a long way to keeping you in riding shape. Work on keeping your back flat. When you get the chance to ride longer especially if you can't do it often It's going to tax you. Do it anyway & enjoy the ride.