A lot of the neat physiological adaptations peak around that 2 hour or 2 hour 30 mark anyway. I found that having some kind of calorie intake on my runs helped me stave off the early feelings of fatigue, but didn't necessarily make me feel like I could run further. Two hours for runners who aren't marathoners is a pretty good benchmark, and I wouldn't go crazy on pushing much past it.
The advice given so far is all good, sound, and safe, but if none of it floats your boat, maybe try the opposite. Get in the first hour of your run nice and easy, and then try turning the second half into a threshold or tempo run. I wouldn't do this if I was already running two hard workouts (which I personally prefer to do, leaving Sunday as long and easy), but one mid week workout and a hard long run has some great benefits. You'll get in some extra miles from the increased pace in the same amount of time.... Or maybe the same mileage in less time, depending on the cause of your fatigue.
Also, maybe increase your weekly mileage a bit? If I'm running 15 miles for a long run, I'm more realistically at 70 or 75 miles per week (as a 1500m guy).
This post being strictly my opinion as someone who isn't really a "long run" guy, and has only put in maybe a few dozen runs between the 17-20 mile range anyway.