Generally speaking, you do not want/need to regularly run your goal distance in training when you get to very long distances. Your body needs a lot of time to recover from extreme distances and you risk sabotaging your training plan or even your race-day-readiness. Additionally, there are diminishing benefits of extremely long runs in training.
Can your run be too long. Absolutely! At what point do you reach diminishing returns where each additional 10 minutes or each additional mile/kilometer no longer provides the same additional stimulus for adaptations as the previous block of time/distance? We can conceptualize the benefits of any run as having diminishing returns after about an hour. Mathematically it’s useful to think of the benefits following a rate constant of 1 hr.
The general consensus is that you get most of the benefit of a run in the first hour, less but still enough to justify a second hour, and less still in the third hour. Beyond three hours, the benefits of continuing are small while the risks of breaking down too much increases substantially, risking over-fatigue that you can’t recovery from within 24-48 hours, which is what we want.
Camille Herron isn't even a fan of regular long runs. There's some evidence presented in that article relating to bone adaptations and long runs.
The evidence suggests that distance running has diminishing returns when it comes to bone health. Troy hypothesizes that bone may respond to the stress of running over the first half mile or so but then become desensitized to the monotonous, repetitive loading. “You’ll get muscle and cardiovascular adaptations, but your bones aren’t paying attention anymore,” Troy says. “You’re just adding miles and potentially accumulating damage, but you’re not going to add adaptive stimulus that will help the bone become stronger.”
The Hanson brothers have a well-known training plan where the "key" long run called The Simulator which is at 2/3 of the marathon distance goal.
“The Simulator” is the name given to a very precise workout created by brothers Keith and Kevin Hanson, who coach the Hansons-Brooks Distance Project. The appellation refers to the workout’s purpose of simulating an upcoming marathon race as closely as possible without stressing the runner to the point of ruining the flow of his or her training. It is intended to put the finishing touches on the runner’s marathon-specific fitness and demonstrate his or her ability to achieve a time/pace goal for the marathon.
The format couldn’t be simpler. The Simulator consists of running 26.2 kilometers (roughly 16.3 miles) at one’s goal marathon pace.