this and this wrote:
As has been stated, the limiting factor for most runners including the top pros is not speed but endurance.
Consider how many high school runners can run a 440 in under 60 seconds. There are far fewer that can string 2 of those together to break 2 for an 880, and a very small number of runners that can put 4 of those 440s together back to back to break 4 for a mile.
This is the most tired agreement in all of distance running.
There are exactly zero people alive or who have ever been alive that had a 400m PR of 60 seconds who also could run 2:00 in the 800m or 4:00 in the mile. So why exactly are we pretending that achieving perfect endurance is just a good training block away?
The example above should be the biggest proof out there that it's never going to be just and endurance problem or just a speed problem. The athlete above is not fast enough to break 2:00 no matter what happens but honestly, their endurance isn't there either.
No training intensity in our sport is an island. Your ability in one narrow facet of running is going to influence your ability in the things that surround it.
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Look, I don't think speed development is this panacea of training. It's helpful and still many have gotten away without doing it. Many successful programs also don't do hill work, or weekly long runs, or CV training, or VO2max sessions, or a lot of other things that some rely on or others thing are at least worth doing here and there.
I think speed development is an easy thing to swap in a few times a month where your strides normally would be. With a good warm-up and gradual overload they add a dimension to your training that is unique and offers a lot of benefit.
I've had exactly one runner injure a hamstring while doing these and it was really my fault for doing it indoors where we didn't have enough room to slow down gradually.