If Jakob wants to achieve his stated goal/desire of holding all records from 1500-marathon, he should eventually do some 20 mile or more training runs to prepare for the marathon.
Gaining "fitness" includes many things, especially when marathon specific fitness is considered.
There is mental/psychological fitness to be gained from running and being out there the long time needed to cover 20+ miles.
There is fuel system fitness to be gained from a greater dependance on fat metabolism gained from continuous long runs of 20+ miles.
There is fueling and hydration efficiency and knowledge that can be gained from drinking and eating during long 20+ mile runs.
There is running efficiency and economy adaptations gained from continuous 20 mile runs at a pace much slower than sub 60 second lap pace.
There are likely intramuscular and running form adaptations that occur to better handle the repetitive impact hardness of the road surface. Roads are harder than modern tracks-although the shock absorbing midsoles of super shoes likely make these adaptations less important.
If one is an elite pro runner preparing to run a great marathon, there are no real good reasons not to do some 20+miles in training.

