I feel that there is a base distance for a runner is a function of capability and attitude. I always that I would run a 10k in my life time, but now aiming for half marathon. This is because, I psychologically crossed a 11k mark one day - which was treated as 10 + bonus... so it has extended to 18 k.
So easily, my base distance is 10 miles ~16k, and on race day, 21 should not be a problem. Some may argue that you should train the full distance, but most people have advice me, it is not required.
Well, these are two schools of thought. I am going to try a full - half marathon ie 21.1k in the next 10 days, so that i know what it takes to finish. Race day is about 19 days away!!!