You can only train at what you want to do. I'd say a guy who can run a marathon at 26mile is 'fitter' than Bolt who runs 100m.
Watched bolt on sky , something I had recorded months ago.(the Olympic sprinter not the Disney’s dog) It was suggested he did 400m but laughter it off as not for him ax training was to hard.
Can’t say he was unfit been the best in world for 12+ years over 200 &100m.
But that said to play professionally football u must be near your piers.
Can’t imagine powell or Gatland only training for 90m or 180m.
Get the point about style and fine margins but you have to be fit enough to combat these different styles.
Never managed to reach these levels but done a lot of sports / fitness over years, on or off and within a few month I can always get my self in to “top of class “