Boredom can make even a short ride seem like an Iron Butt one. For me, it's simple: I ride as long as I'm having fun. When the fun stops, I stop.
Finding some place interesting to ride helps immensely. Starting the ride early in the day, well before sunrise perhaps, gives you the advantage of getting out of your home area while it is still too dark to see anything anyway, and saving the daylight hours for new places that might interest you.
Initially, instead of seeing how many miles you can do in a limited time, try seeing how many hours you can ride without getting bored and tired of riding, without any mileage goal. See if you can stretch the hours out first, the mileage will follow eventually. That is much safer in that you will not be tempted to ride faster to reach a certain number of miles. It will also let you get off to take photos, eat a snack, stretch your legs, etc., which will help you to last for more hours.
One of my favorite places to do Iron Butt rides is the Dalton Hwy. It is interesting, scenic, somewhat challenging, and never boring. It is no trouble to ride my KLR for 30 hours or more when I'm up there, getting off now and then to snap a photo, then back onto the bike and keep moving. When it is no longer fun, then I'll quit those rides and look elsewhere.