How long until you see the benefit of increasing mileage?

If you built slowly, you should already be faster. If you built too quick, you'll be injured before your race.

Right.....but in a build you usually don't race a whole lot and competing against your easy day run day-in-and-day-out seems like a recipe for injury. I think the question is still valid.

A long time. You might see gains here and there, but ultimately the real gains will take a while.

How long have you been at 50? If it's your first week, expect fatigue in about 1 or 2 more weeks.

If you increase mileage slowly and allow yourself proper recovery (e.g. diet, sleep), then you will see benefits within a few weeks. If you do a workout on 20mpw and then repeat the exact same workout a month later after weeks at 25/30/35/40mpw, you will notice that the second workout was far easier.

It varies person to person. More slow twitch athletes are high responders to volume and will feel fitter more quickly. They also tend to do better with short tapers, keeping volume pretty high until a few days before a race. Faster twitch athletes are the opposite, get more bogged down by volume in general, and need to build slower and will take longer to see improvement. Especially in easy pace. Personally I can't rely on using easy pace at all because it varies a ton and often gets slower for a while when increasing training

And for 800 meter runners......................it's complicated.

very quick, if no improvement i would go down in milage and would make sure to run 2-3 quality runs per week first. You should see improvements in these session quiet often.

Then add as much extra milage as you can do while not compromissing the quality of these workouts.

Example:

I used to run 2 workouts/race per week (we,sa) on 20 miles and then added more easy/steady runs to run 35 miles. Keep running PBs every 2 weeks in races, this continued untill i hit 45 miles. I then got slower when i tried 55 miles as I got to tiered todo the workouts, so i went back down to 45 miles started to improve again...

Milage is support for your harder runs/workouts and only do the minumum amount of training that you need todo to improve. More milage also allows for bigger/harder workouts, so add some milage then make your workouts harder before adding more milage...

About a month, but there are a lot of factors at play, so it's going to depend. Considerations include, is the the first time you have jumped from 20-50? How long did the build-up take. What kind quality work are you doing? Age?

And note that there might be setbacks and plateaus along the way. If you get up 60-65 the benefits will continue to accrue for another year or even several years.

A couple months

It’s all about the time you spend at said mileage.

When your body adapts to running 10 miles a day and running long grueling workouts, races feel a lot shorter and your body feels a lot more efficient holding race pace. As others have said, the improvement is gradual, and it kind of sneaks up on you. When I’m running 70s during base there’s always that day where I finish a run and I’m like “wow, I just did 13 miles and my legs never hurt and my HR stayed in the 140s, I must be fit”