A lot of people are focused on the 'hardship' aspect of this, but I think the toughness that comes from daily cycling is a bit more nuanced than that. I ride every day in part because it makes me a more useful human being: I am able to transport myself everywhere that I regularly need to go with very minimal reliance on external assistance. My daily functions are not subject to whether the bus will be on time, or the cost of gas, or road conditions - I can be responsible for my own business, which I think is a key trait of capable, successful people, and one I perceive as rapidly dwindling in the developed world.
Like everybody else here, I've had coworkers say that I 'must be tough' to be riding on any given day, but I don't see it that way: I'm just continuing to develop myself. All the stories people have mentioned here about their impossibly tough ancestors are impressive less for the physical prowess they suggest than the sheer dynamism of people who, confronted with hardships, just developed and adapted themselves to overcome them. Daily riding has adapted me: made me healthier; stronger; capable of longer endurance; less fazed by external factors like weather, darkness, traffic; and more self-sufficient than I could have been if I'd continued driving to work and struggling to make it to the gym twice a week. Toughness is an aspect of that adaptation, but it's not the whole story.