Pro-athletes are probably a bad example for anything other than being pro-athletes. I have got the impression from some of the interviews with swimmers and tennis players that they utterly loathe training with a passion especially when their way of life is was designed for them by their parents before they ever had any self-direction.
Maybe not 'the', but 'a'?
What do you think "the hallmark" is?
I don't think anyone said anything about 100+ hours/week. I was hoping that commenter #2 might appear and give us some deep insights, but that appears to have been a bridge too far...
I don't have a 9~5 working for the man, but there is very little that happens during my waking hours that I wouldn't classify as related to work. I'm fortunate that this is not one task, so it doesn't weigh on me like fighting with a C compiler while sitting under florescent lights in a cube farm might. That's kind of work is fine - I like that too - but I couldn't do it for more than about 70-80 hours/week.
My simplest answer is to pick the right goal. You know its the right goal when you can envisage countless possible setbacks but it would seem absurd that they could cause you to abandon your goal. Give up on reaching the Olympics because you run out of money, have a bout of flu, or a parent dies? It should feel like a non-sequitur. Your goal needs rise above the inevitable twists and turns of life - it should not just be a "fair weather" goal you would drop when clouds pile in.
Your interest in the goal should be deep. It should not be for the moment it can be said it is done but for the transformation that achieving that goal brings. You must not simply want to reach a finish line, you have to want the life that comes after crossing that finishing line. If life after reaching a goal is little different or you are indifferent to that life, it is a weak goal, it won't sustain you through the hard times.