%FT + %ST = 100%
The best sprinters are gifted with the most FT fibres (FT = 62%) and ultra distance runners have the maximum amount of ST ones (ST = 82%) [souce: this book, p.18]. By the pigeonhole principle* your body's muscle composition must be perfect for some distance in between. Now perhaps your 'ideal' distance is non-existent, like 17.3 kilometers, but who cares? You are absolutely perfect for it. You were born for that distance. You are an ideal specimen for distance X. Even though I have never met you, the statement is an undeniable fact. You can also change you ST/FT ratio slightly depending on your life-long exercise regimen, but what I'm stating here makes the point moot.
Some who like running marathons are told they don't have the 'genes' for running that far. Perhaps such a person wished they too had been born with more endurance-specific muscles. Sprinters on the other hand, the ones that wish they could run ever faster, envy others who, they are told, have a high percentage of type IIa/b FT muscles. They might wish they too were gifted with a high percentage of sprinting-specific muscles. Hence some are gifted sprinters, others gifted marathoners. No-one likes to learn they will never be good at something because they weren't born with the right 'stuff'. But you were born with the right stuff, only you might discover your perfect distance is something a little different than you expected. You only have to think outside the box, just a little.
If you follow the early career of professional athletes, many switched between distances after they found the one they 'loved' were not ideal for them. Examples abound. Prefontaine moved up from one mile to the two, Ussain Bolt dropped from 400 meters to a 100m/200m combo, and David Rudisha moved up from 400m to 800m. Each was from the advice of their respective coaches. Many runners also switch from 10k to the marathon (or vice-versa) and so on. Brad Hudson, in his book Run Faster points out that some of his athletes work best as 3k/5k runners while others are 5k/10k specialists. And Ann Trason finds marathons too short for her liking (or so what I read in Born to Run). The evidence is there in plain sight: everyone, including the very best, are constantly experimenting to find which distance is ideal for their bodies.
Update: Here is an old article that sort of discovers a corollary to this idea of what a 'perfect speed' is:
It is interesting to note that when oxygen consumption is expressed relative to body weight, there appears to be an optimal running speed for these subjects. Using the mean oxygen consumption values for each speed observed in the present study, two distinct regression lines appear evident. The intersection of the two regression lines occurs at approximately 185 m/min, or 11.1 km/hr.What is of particular interest to me is that this 'optimal' speed is much slower than what any of these trained subjects can race at (the study focused on trained runners). Even more amazing is that untrained subjects have an almost identical 'optimal' running speed of 11.0 km/h. That means a trained runner does not improve his or her optimal running speed, but instead improves their optimal power output. This is a very interesting point I don't hear discussed often, if at all. The formula for power output in runners is not a simple equation, I expect, for it depends on your muscle fibre turnover controlled by your brain, which is not well understood. In fact I know of no way to accurately measure this aspect of running.
*Aside: I love pigeonhole proofs. Like proving that at least two people in any city with more than a quarter million residents have the exact same number of hairs on their head. And without any counting of hairs. How cool is that?