Okay, first off, high mileage is relative. And the numbers I'm using won't be considered high by most standards.
In my case, I was running 35-40 miles/week as an 18-year-old and ran 37:xx on a 10K course. I bumped my mileage to 60 miles/week and, 2 months later, ran a 34:27 10K. I increased the mileage to 70 and ran a 33:59.
I held 70-75 for several months, about a year, and ran a 32:20 10K with nothing more than mileage and running a 1/10-mile hill 20 times twice a week. After adding speedwork, I ran a 31:50 for my first track 10000 and added a 30:08 10K on point-to-point course that had a tailwind.
I tried 100-mile weeks, and I ran fairly well with them. However, I mentally wore myself out. In hindsight, the problem was that I was trying to do too much with the 100-mile weeks. I was trying to get the mileage of a marathoner, the speed of a miler, and racing nearly every weekend while trying to balance the demands of being a employee, husband, and father. Overall, just a little too much stress. I would have been smarter to put in the miles to get the base and then drop the mileage for the speed.