The point is that the droplets persist in the air for a little bit, and the faster you're moving, the greater the likelihood that you're moving through a space that still has suspended droplets in the air. It's like how the faster you drive, the longer your stopping distance. Your safety distance increases the faster you're moving. Six feet is for when you (and everyone else) are stationary.
Running doesn’t (substantially, probably) increase the rate at which you shed the virus, though – so the droplets are spread over a larger area, at a lower concentration. You need to keep a greater distance to guarantee zero exposure, but that doesn’t mean that higher speeds equal greater risks. The car analogy is inappropriate.
Yup, and when running you're breathing deeper and more forcefully, so you're expelling more viral particles farther.
Plus, I'm not seeing as many runners wearing masks (no doubt because of the high oxygen requirements of the running itself), so that too has an effect.
