This has an effect on the rider's efficiency (meaning it affects the rider no matter the terrain) but not directly his attributes (in fact that's not a such important point, but during the race we rarely modify the attributes -except maybe for injured riders who hit the ground hardly).
< 150km => no effect
> 250km & more = same effect
so everything is between those 2 values.
150-175km : only riders with END < 60
175-200km : only riders with END < 70
200-225km : only riders with END < 80
225km & + : all the riders
The malus is progressive, when you start being affected you don't feel any difference but few kms after it starts making you less powerfull.
(so nice job Kentaurus, you have some good conclusions in your topic)
for instance: a rider with let's say 65 END will start to have a micro malus at about km183 in the stage, and from this point his malus will increase slowly till the end.
of course his malus at the beginning is nearly 0 so you can't have the perception at this exact point.
don't forget the END is not always the END you see in the UI (depending on Day Feelings, Fitness...your END can be different from the value displayed in the UI).
as always with the attribute's values, it's important to keep the same "scale" as the original DB (to avoid fucking up the whole game with all the side-effects processes). this point would definitely desserve another topic (not today :redface to explain you how important it is not to do "magic" with the numbers and what could be the effects when you think you'll make the game better with a new "scale" of values.
so if you have suggestions about changes on the END part (for the future) you can discuss it here.