I watched a bunch of videos of people trying to maintain Eliud Kipchoge’s 2 hour marathon pace on a treadmill where you can set the treadmill to hold a specific pace. I’d love to try that. (I’ve tried it outside but I can’t hold a steady pace at that speed so it’s hard to gauge if I’m going fast enough. Assuming that I can even run that fast!)
Could you try using a stopwatch on a 400m track? Just keep pace in 100m increments -- you'd have to cover it in 17 seconds for a 2 hour marathon. I'd personally prefer that to a treadmill, because I have never even tried to put a treadmill up to 13.1 mph (do they even go that high?!), and I'd be worried about flying off the back of it and hurting myself. Also, having high enough leg turnover to keep up with a treadmill belt moving beneath you is not exactly the same as actually propelling your whole body forward at the same pace, so doing it on the track would be a truer comparison, to boot.
As for the treadmill boredom question... I'm perfectly happy running in loops forever (either I'm spaced out in a meditative state, or I'm working too hard to care), so boredom on or off the treadmill is not an issue. But my main reason for wanting a treadmill is for backup in case of inclement weather. A blizzard can bury (or ice up) the local sidewalks, shoulders, trails, and track for many days. A long run scheduled during a summer heat wave can be very dangerous. If your feet get soaked through, you can wind up rubbing the skin right off your toes (ask me for my gross photos, I have them, LOL). And for the non-morning people among us, it's convenient to be able to run after dark without risking getting hit by a car, tripping on a curb, or eaten by a coyote.
Treadmills are also nice if your neighborhood has a lot of crosswalks or traffic. The incline feature lets you do hills if you don't have any of the appropriate grade nearby. (The opposite can also be true -- you can be surrounded by hills, but you just want to do some flats.) You can also watch TV while running, keep ample food/drink nearby, and take unlimited bathroom breaks. I believe the belt is also easier on your joints, at least compared to concrete/asphalt.
So there are plenty of reasons to go treadmill, but it's ultimately down to personal preference. But I'm totally with you on the stationary bike, though -- I can't stand them.