If you want to find out what works and doesn't for improving running performances you're much better off looking at "anecdotes" from people who've gotten results, good or bad, than looking at some so called scientific study done on rats or with humans over a span of a few weeks. You're seeing things that actually happened rather than at things someone believes should happen based on studies done in partially similar situations.
bigger than a weight wrote:
Too much training can hurt sleep. It can make you tired. It can make legs rubbery. It negatively impacts workouts. Some people run best at 40 MPW. Some at 60 MPW. Some at 100 MPW.
Almost everyone’s optimal training volume (assuming their only priority is running — which of course it usually isn’t) is far higher than 40 MPW or even 60 MPW. But most people try to maximize volume AND intensity at the same time, and that is rarely a good idea, and so blow up long before they reach their optimal training volume. The vast majority of runners never even attempt a reasonable build to even consistent 70 miles a week, and so never discover its benefits.
In response to the OP’s question, of course there is an upper limit, but 99% of runners never even come close to learning what that is, because they make mistakes along the way that lead them to believe nonsense like 40 mpw is their maximum optimal training volume.
Elite runners are elite because they can handle high mileage. 90% of the population is not made to run. The majority of average runners can't get past 40 miles.