It takes a lot of time.
You need good support at home for the meals.
I'd done a lot of 70 mile weeks.
What got me over 100 was it mediately drinking Endurox, a carbohydrate protein supplement after each run. Big difference in recovery. Highest mileage was 108.
My dad was dying that year.
I ran a marathon and the JFK 50-miler that year. I've done two 50 milers. In each case, my resting heart rate before the competition was 40, and after the competition it was elevated 20 beats at 60 for 3 weeks straight.
As far as helping me to my best performances, I got the best results maintaining 70-75 miles a week for 4 to 5 months straight on soft hilly trails. 3 miles grass in the morning, 7-8 miles trails or grass in the evening, only 8 miles easy on Friday, 16 to 18 hilly miles very easy with ultramarathoners on Saturday, super easy 5 miles on Sunday. About 90% on soft surfaces.
I have also crossed train by running on the bottom of a pool in 4 feet of water, forwards and backwards for up to two and a half hours. For that I counted a 100 calories as one mile.
That's all for now. I've been running for 45 years.