It seems to take a work-like devotion to achieve K-Hound and for me, although I cannot remember a year that I did not ride more than the 10K and many years rode more than double the distance, it just seems like a stressful chore. Apart from SR rides, many of my weekly training rides were 3-4 hours long or about a Populaire done 2-3 times per week. A dose like that seems manageable in terms in minimizing family disruption and negative impacts to health/fitness. The other thing for me is cost. Traveling to brevets can get costly whereas jumping out the front door onto a Perm is basically free. I used to have brevets close to my house and a 200k did not cut into family/weekend but if you add 90 minutes of driving on each end of the ride, the balance starts to tilt the wrong way if you are trying for 10K. I would guess Populaires are popular owes to lower stress on the body, wallet, and family time. A 400k or 600k is a real test and eats into one's reserves and I cannot imagine a steady diet of them is beneficial. I can think of at least 4 very well know endurance coaches who say the longest routine training ride should be 6-10 hours and that is for RAAM and TCR type athletes. With that my guess is 100-150km would be a good distance for someone trying for K hound. But, I never did it and what do I know.