The Endurance skill lowers the stamina cost of sprinting.
Each "tick" of sprinting originally would cost 20 percent of your total stamina. Every 1 point you put into endurance takes .01 percent off of this total per tick. If you have 100 points of Endurance, this is reduced to 19% per tick.
To make this skill effective, one would want to add to the total number of Sprint ticks.
100% of your total stamina divided by 5 ticks of sprinting = 20% (The original amount taken away by sprinting)
100% of your total stamina divided by 6 ticks of sprinting = 16.67% (Which would require 333 points of Endurance)
100% of your total stamina divided by 7 ticks of sprinting = 14.28% (which would require 572 points of Endurance)
100% of your total stamina divided by 8 ticks of sprinting = 12.50% (which would require 750 points of Endurance)
As you can see, if you begin sprinting from max stamina, 750 points will get you 8 ticks of sprinting, as opposed to 5 ticks with zero points invested in Endurance.
It can be argued that having more than the exact amount required to gain an extra tick of sprinting would be beneficial. This is because there are many times you do not begin sprinting from full stamina, such as in mid combat.
It should be noted that you get small stamina regen ticks while sprinting, usually around 1-5 points. Because of this, you will be left with a few points extra to give you an additional second of sprinting. So, with 0 Endurance, you can actually sprint for 6 seconds, not 5, because of the fact that you will have 1-5 Stamina left over to get an extra tick of sprint. Likewise, with 750 Endurance, you get 9 seconds of sprinting. ~Latsabb