Does that mean you never came back? Is doing half a run like digging half a hole?
Only ran half the prescribed route...
Every run, considered in itself, is a whole run; but it may be only half of some other actual or possible run, such as the run that Clint should have done.
I think maybe J-J, given his eye (or ear) for paradox, should have been a philosopher.
I think what happened Saturday was several runs, then.
Thursday's run, as it was occurring, was indeed the run that I should have been doing. It was only after I terminated said run that it became something other than originally defined. By that logic there is no DNF, there is only a redefinition of the intended run.
There reached a point in Saturday's run(s) where I determined that I felt quite fine and that the probability of my feeling the same level of fineness 24 hours in the future was rather low, so the run became redefined. But did it become a second run when I didn't even break stride?
Perhaps J-J is alluding to the ambiguity in 'half'. One run may be half of another in virtue of some implied unit of measurement, usually distance. So the run I did one day is half the run I did another day if it is half as long. Or one run may be half of another in virtue of being literally a part of that other, as when we say "the first half of the run went well".
One might wonder, on this latter sense, whether every run is many runs, since every run has infinitely many continuous stretches as parts (assuming the continuity of space and time). But in ordinary contexts 'run' refers to the single maximally continuous stretch of running, so that we can truly count runs, and say, for example, I did two runs today, one in the morning and one in the afternoon. It's similar with many of our concepts. Look at an iron sphere and ask, how many iron spheres are there? In ordinary contexts, the answer is one. But of course embedded in any iron sphere are many smaller iron spheres that are not maximally continuous stretches of iron.
For my downhill skiing, of course, it's unambiguous provided I don't jump off the chairlift.