No question in my mind that the sub 15 is the hardest, then the sub 1:10, then sub 2:30. In he last year, spanning ages 45-46, I have run 15:41, 1:11:04 and 2:30:43. I was three cramps away from sub 2:30. Sub 70 would have been a reach on a very good day where everything came together. Sub 15:00 wasn't happening. And I am a natural 5k guy, with that being my best event when I was a less mature athlete.