In many respects it is the endurance component that defines distance running.
In order to improve that endurance we gradually increase the distance of our longest run. The point where we stop increasing the distance is a factor based on training history and the distance of your chosen race.
The stimulus in terms of distance that would be applied to a 5k runner may be an 8 miler but runners training for the Comrades Ultra Marathon may need to run say 32 miles in training; certainly that was my longest run for that event. There is a Pareto type Principle (the 80/20 rule) with endurance training as the purpose generally is not to just cover the distance, nor is it to go too hard on these runs as you want appropriate adaptation but not a lengthy recovery so you can get the consistency of training in.
A good question to ask is if you are running at an appropriate pace and intensity. It is often reported for example that long runs should be 70-85% of max heart rate. So if you are in this band then one is facilitating the adaptations you are targeting adaptations, which is the point. If you aren’t, then are you stuck in “mid-pace mediocrity” where you are going slightly too hard on your longer slower runs yet hard runs are not hard enough?
Remember the mantra: Make your hard sessions hard and your easy sessions easy.
With some of the athletes I work with I find a great way of conducting some of our long runs is to set very specific paces. So if you run a 3:03 marathon (7:00 per mile pace), then we might do a 12 mile session where after a warm up you run 4 miles at 7:45 pace, 4 miles at 7:00 pace and 4 miles at 7:45 pace. We are aiming for adaptation and in fact this is a nice race pace type session. I think that by targeting these specific adaptations and understanding pacing you can avoid any mid pace grey area work that does not bring the adaptation you desire.
Run smart guys!