Its not like these players are punished for not completing it, its as much a mental challenge as anything. I dont think there is any player headed into the offseason training to run marathons vs training for the season. It can feel a little weird running in those circumstances but its not going to kill them and like you said 6 mile runs are doable and not some ridiculous expectation, its just motivation towards keeping them honest.
It leads into todays skating drills too. I watched a little of it and you could see who was hurting from it. Cam, Nuti and Savard were doing some drills together and Savy was skating circles around the other 2, thats gotta be some bragging rights as thats never happened