I had a friend who ran 2:17:01 and he barely approached marathon pace for his workouts. He just ran by minutes and would run really slowly. A typical long run was 2.5 hours-3 hours at around 7:00 pace and he would aim to get at least two hours of running a day. He'd occasionally jump on the track for some random and unstructured intervals but he felt best with a lot of easy jogging.
There was dude from the 70s who other than doing some occasional mile reps and racing often, just ran a lot. 130 mile weeks were common. Ended up running a 2:09 marathon.
When I was 49, I was doing as much mileage as I could and not much speedwork. Averaged 60 mpw. Ran a negative split 3:13 marathon. The following year I dedicated to speed racing miles, 5ks, and everything up to a half. Had no intention of running another marathon but my race times said I was close to 3 flat. Went out in 1:30:30 and finished in 3:08. Positive split 7 minutes and was still 5 minutes faster than the year before. I was running 35 miles a week and only did one run over 13 miles.
It's not mileage or speed. It's the right amount of both to get the best result.