Maybe there was some tongue in cheek involved before in my previous post. But let's get serious for a moment and re-evaluate some of the comments in the original post.
The question is why there are no multi-day running events, like the Tour de France. In fact there are many, and popularity of such events has actually resurged (relatively speaking) since 1980.
The suggestion was that these kind of events would increase the popularity/awareness of the sport. I think it is evident that current multi-day running events have not done that in the slightest. In fact, while these multi-day events have come back, track popularity itself has declined. Maybe it's the "ultra" aspect of the races, and it's a good direction to shorten the races, as you proposed, and inject some kind of mass marketing campaign. But I interpreted the "like the Tour de France" part, to represent longer events like the ultras rather than short races like 5K, 1500m, and 800.
I would argue that Yiannis Kourous dominated multi-day events much more than Lance dominated Tour de France, yet it is clear one is a virtual unknown, even to most running fans, while the other one is a household name, world renowned super hero (or super villain) known to cycling fans and non-cycling fans alike.
While such a multi-day running event could be interesting to a group of running fans, I think it is optimistic to think it will capture the awareness and popularity of the general public, or that it could eventually hope to compare to bigger events like the big cycling tours.
Adding one more thought, it's kind of funny how the big, long tours in cycling are popular, compared to say track cycling, yet for running, the big long ultras are virtually unknown, when compared to running events at the track (which themselves are struggling in popularity).