12 mph is fast as shit and much faster than any non-elite runner can sustain in an outdoor run either on a track or a paved surface. 12 mph = 5 min mile = 15:32 5K time.
For context my fastest ever 5K time was 17:39. I did that as a 10th grade HS cross-country runner with very good but not elite running ability. That race got me a spot on the Varsity squad. 17:39 = 5:41 min/mile pace = 10.4 mph. And that was bottom level Varsity HS time for skinny kids that specialize and train only for 5K runs. When I was that fast I was BARELY able to squeeze out one 5 min mile = 12 mph for 5 minutes if I just went for broke over 1 mile.
After 11th grade I switched focus to grappling and strength training, but continued to run frequently for general conditioning. As a 160+ lbs adult, even with 5 days/week training runs geared towards recreational 5K's, I've never been able to break a 19:10 5K = 6:10 min/mile = 9.7 mph. I can tell you that running that fast in any public place (track, park, street), will make you 99% of the time running faster than anyone else in the area and usually much faster.
EDIT: I'll even amend and say that if you can sustainably run 10+ mph you will almost never, if ever be outrun in a public place during training. In the last 25 years doing training runs outdoors, I've never been outrun over the course I was running. I wear a HR monitor watch with GPS. Even doing training runs in Central Park in NYC (where there are many fast runners), 99% of the time I was faster along the course I was running. And the 1% of the time when there was some super fast asshole around, I would make it a point to haul ass, crank it up to briefly to around 12 mph and pass his ass before hitting a fork in the road and turning a corner to go a different way, so he wouldn't see me panting and wheezing behind a tree LOL.