e.g. the 1743m race. or the 19 mile race. Basically, if you lined up with all 6.5 billion people and said "let's run X," this would be the distance at which you had the least people beat you.
Also, which actual race could you run at WR pace the longest, percentage-wise? (note: not the actual splits of the WR run, the average pace) And just to be clear, the 100 isn't a good answer just because at the end you wouldn't be that far behind, because I highly doubt you could ever get up to 100m WR pace.