Invariably, on the treadmill, I run a mile, and it's fine, and I could keep going, except that right about then I die of boredom.
Perhaps it's the lack of landscape. Or the odd bounce of the belt. Or the steady, rather insulting red numbers. Or your tendency to get your arm caught in your headphones and toss your iPod across the gym.
Or the knowledge that, for all that effort, you have gone literally nowhere. You are a hamster. Your life is an exercise in futility.