optimisticman wrote:
I you could hold 72 s / 400 pace for a marathon you'd be world class (~ 2:06:30 ). Speed isn't everything. Speed endurance matters, though.
That's not quite true.
World class distance runners like Kipchoge can run a marathon at about 18-20 s slower each 400m than their personal best (69-70 s compared to maybe 50-51s).
Add this to your 72 s and you end up with 2:38 to 2:42.
But more realistic is not to add the total 20 s but the relative amount of 40% (20 s/50 s) which is about 29s for 72s. That leads to a marathon time of 2:57.
So you can assume the OP's potential marathon time is limited to about just inside three hours even if he trains very much for endurance.
However, within the training process he would maybe get a 68s 400m PB which is more in line with his 15.5 100m. This immediately opens the range to sub 2:49.
Every second gained in 100m would provide another possible 11 min gain in marathon time though.
Hope you understand now, why speed is important for all distances.