science prooves its harded to run 18:00 for 5k than 15:00!!!

What's harder a 3k or 4k? They are both hard -- if anything I find the 4k to be easier.

Among other things, pain tolerance correlates with performance in running. So, if you select a random sub 15:00 5k runner and a random sub 18:00 5k runner, the sub 15:00 5k runner is more likely to have a higher pain tolerance (be that an arbitrary measurement) than the sub 18:00 5k runner. Therefore, the sub 15:00 5k runner is more likely to be running through more pain than the sub 18:00 5k runner.

ok well this 1500 minute guy can run 6k in the time it takes you to run 5k

So which would the faster guy find more stressful that 5k or 6K? The longer the duration of a race the lower the intensity (fraction of VO2max) at which you run that distance. So racing a 6k is lower intensity, but for longer time, if both are run by the same individual and both cases are all-out efforts. You need to compare duration of a run to compare intensity, but you need to compare distance of the run if you want to compare total amount of work done. So the 15-min 5k guy is working at a higher intensity, but for less time and the 18-min guy is at a lower relative intensity, but for longer. Take your pick as to which feels the stress of the race more.