Two interesting points of discussion:
First the ten hour test. Obviously, there is a fair amount of variability between different types of runners even if those runners are generally running around the same time- some runners need more mileage to get to the same place as other guys who can get there on lower mileage but higher quality and who may not have the mechanics necessary to handle higher volumes of work. That being said, when you back into the math, it just seems intuitively right. Seems like a pretty good guideline for estimating a baseline, long term mileage goal. Some folks will be able to run more hours, some less, but I suspect if you took a pretty broad cross section of experienced runner of different abilities who feel like they are training at or near their limits, the bell curve would be pretty tall right around that 10 hour mark.
I would be willing to bet that the bell curve around the 50% test would be a lot flatter and a lot wider. I just get the sense that thee variability in runners is a lot greater here, with some runners (like me) getting out of running shape fairly easily, but being high responders who can come pretty close to their ceiling after a relatively short period of time and then really needing to labor away for extended periods to see much more improvement, and with other runners (like Stonecutter) who probably maintain a fair amount of their fitness even when not truly engaged in training. The variability in natural ceilings, time it takes to approach that ceiling, amount by which you regress when detrained, etc. is huge and it is what makes watching runners progress over time so interesting.