Sep 17, 2021· Grindstone 100. Grit, endurance, temporary loss of sanity. You might need all of these if you want to attempt Grindstone. If you want to finish, well, just keep in mind this is, without a doubt, the hardest 100 miler east of the 100th meridian. Now that you're hooked, Grindstone is going to be an incredible adventure for each and every entrant.