Why is the 400m the marker of speed anyways?
It takes strict talent to build the speed-endurance to maintain "speed" for 400 meters. This is more of a indicator of anaerobic power, which is the ability to QUICKLY accumulate lactate to a high level.
For instance, Bolt opens the 400m in 45.XX but has FAR superior speed than Kirani James who'd run 44.xx. But Bolt, at least currently, does not have as much endurance to maintain speed as Kirani would have.
As for distance runners, it is definitely a bit counter-intuitive to build this to a high level. That would take specific training, which would focus on anaerobic development rather than aerobic development. Instead of focusing on 1k's at 5k pace early in the season, the miler would need to focus on 250's near max to fully train the 400m talent. 1k's @ 5k pace would decrease the ability to run a fast 400m...
For instance, Steve Cram was an "aerobic beast" of an 800m runner, who only had a 49.XX 400m time.
More or less, speed could be measured in top-end speed in perhaps a 100m fly than a 400m standing start. It takes into account pure top-end speed rather than speed endurance. And with this being said, that won't indicate too much as shown by Mo Farah vs Galen. Finishing Speed vs Top Speed...