Zero.
If you're a racer in a marathon, you'll train to a speed bias and survive the miles in the race.
If you're a participant in a marathon, you'll train to mileage bias and finish the race.
Younger people (<30) should be more biased to speed. Into the 30's and up, bias will be towards a balance or mileage especially as top speed changes relative to race pace choices.
If you want to go beyond 18 miles in a day, split the workouts.
After 18 miles (or so) of running you're burning muscle. Longer recoveries required.
Race to taper and recover with fast speed and long distance on race day. That saves the body over the years.
Triathletes can substitute the long runs with bike and swim for aerobic stuff, and weights for strength.
https://www.palmtreesahead.com/ https://www.palmtreesahead.com/tactics2faster-new