Hill I'm willing to die on - Distance runners should do legitimate work on speed/power year-round.
By that, I mean distance runners (800-marathon) should have a day of training every 10-14 days or so where raw top speed, muscle fiber engagement, and/or foot-strike (minimizing ground contact time) are the primary focus. I'm not talking about strides. I'm talking about very short, 100% effort sprints and/or steep hills, along with plyos and/or heavy lifting.
I know a lot of distance runners are hesitant to do this, because it usually means a "low-mileage" day. A lot of runners also hesitate to do this because, if done properly, it is a day of high stimulus on your neurological and muscular systems, which means it should be treated as a workout day. That means "sacrificing" a workout that we are more comfortable doing.
But I thoroughly believe it is worth it. I have seen so many distance runners (including myself) who work so hard to improve their basic endurance, their thresholds, their vVO2, etc, start to struggle and start to get less out of their hard work than they should because, as their fitness increases, running at the proper intensity level becomes disproportionately stressful on their legs.
Being purposeful about training your legs can help unlock all the other work you do.
Oh, and those that are concerned about mileage, my experience has been this - if you devote a day (and a workout) every 10-14 days to your legs, your mileage might dip initially, but within a couple of months, your overall mileage for each of these 10-14 day periods can/will be higher because working on your legs will enable you to handle more on the other 9-13 days in the cycle. You will also be able to handle volume in your other workouts, so your overall "workout" volume scarcely suffers, either.