Back in college, I had a roommate who would go for hour-long runs two or three times a week. One night, I spontaneously decided to go running with him. It soon got so painful that I had to stop. I was only able to run for fifteen minutes that night. When he returned to the dorm (after completing his run), I asked him how he could keep running like that; did he not feel the pain? His simple answer surprised me. Yes, it was painful, but he endured it.
Today, I registered to run in the Seattle marathon.
I've never tried running a marathon before. Or even a half-marathon. But I didn't sign up for this because it was going to be easy. I signed up for the same reasons that I leaped off a 40ft high bridge. I signed up for the same reasons that my friend tried out for (and got accepted into) the Navy SEALs. I signed up for the same reasons that the United States chose to put a man on the moon.
I signed up because "we do what we must, because we can".