Staying fed/hydrated, moderating intensity, and not overthinking it. Riding more miles isn't hard. Figure out your endurance pace, and you can ride all day at that pace. My level of fitness is wholly unremarkable, but I still churn out 200 miles a week, every week.

My wife is lucky to get in 3-4 rides a month, but can go cold into 60+ miles and finish it just fine (so long as it's not too hot out.) She hadn't done a ride over 30 miles in over 6 months, then did 100km with me on my birthday, and a month later 102 miles on our anniversary.

People blow themselves up on longer rides by trying to keep up with people faster than they are. Ride at your own pace and 50, 60, or 70 miles is no problem.