Short answer to why elites dist runners can't improve their 400m time much: They can really only improve their start time. Experts - how much does a faster start impact 400m?
Longer Answer:
The average person (50:50 fast/slow) can only recruit about 50% of their muscle motor units, but a well trained mid/long dist runner has an exceptionally efficient nervous system that can invoke a much higher percentage of motor units. The body is lazy, it only recruits the small slow twitch units first and the fast twitch last. The super efficient distance runner will not only recruit more motor units (90%+) their recruitment will dip into more fast twitch units than Mr weekend warrior or a muscle bound sprinter who is poorly trained.
[side note: This also explains why after 1 or 2 weeks of weight training you can lift significantly more weight: you haven't grown much, if any, new muscle, but your neural factors just more efficient at invoking more muscle motor units.]
About 40% of fiber type is due to environmental influences (training), 45% genetic. You can convert between Type IIB (glycolytic) and IIA (Aerobic), some say you can't convert between Type I and Type II, some say maybe 10% conversion. But many feel fiber type is mostly overrated except for elite pure sprinters. Here's the thing, all fiber types have the same force! Just that contractile time for IIA is twice as fast as TypeI, and IIB twice as fast as IIA. Of course the TypeII will help with your start, but once you get up to cruising speed in a 400, does the faster contractile time matter? I don't think so, you're no longer accelerating.
So, my guess is that a well trained dist runner trying to become a
400m runner won't improve much, because their neural factors are already optimized. Since all the fiber types have the same force, the only thing they can significantly improve is their start time, by biasing their training to Type IIB.So, how many seconds would you save in a 400m by optimizing your start time?
Refs:
http://www.higher-faster-sports.com/muscletyping.html
http://www.higher-faster-sports.com/fasttwitchmachine.html
https://www.outsideonline.com/1783586/it-possible-change-my-muscle-type
https://www.t-nation.com/training/secret-to-motor-unit-recruitment