Well, 1st I think they would have to pass the definition of “runner”, which I think many so-called runners do not. I think you can be “hardcore” for a short period but to put in years and years of training with the aim of actually improving as a runner is a truer defination. That would require a love for the sport, which I would think, or hope, every hardcore runner has in spades. Hardcore requires you to prioritize your training. That inspite of your schedule or unexpected “stuff” you do not let life control you. This will require you to posssibly run at odd times of the day and in all kinds of weather. Which all goes back to the requirements of a runner; to be adaptable, durable and resilient. To have a higher than normal level of mental and physical toughness. The same toughness you need at the end of a race is the same quality that is cultivated everytime you step out the door. Hardcore implies an extreme association. If you find other runners to shake their head at your training schedule you may just be “hardcore”.