LSD - or long steady distance is the key to good endurance. You're not looking to race ididaride so moderate pace is fine. Long slow miles will make you great at riding slow...think of it as steady...keep the same cadence, speed, and effort. Start off in blocks 1 hr, then 2 hrs, ect....or whatever is comfy for you as a starting point.
if you can monitor your HR then Stay mostly in zone 1-2 (you'll go over these on any kind of climbing, but dont worry about those short efforts). get 50% or more of our ride in zone no matter how long you ride and you'll see results.
Also focus on time in the saddle and not distance... I usually take the distance off the display for these rides as distance doesn't matter. be sure to bring nutrition with you, whatever works be it water and snickers or hammer endurance drinks and gels.
that's about as secret as it gets for endurance...
just effffing ride.