I was a horrible high school athlete, not obese, but not trained, rather played video games and studied, couldn't finish a proper 800m without walking.
Started training on my own at 18, mainly with weights and did intervals with bodyweight exercises. Have never run a 10k in my life. Have run 5k maybe 20 times a couple of years ago, time was around 22minutes towards the end if I remember well.
At the moment I am 25, just a hobby athlete, decided to run a bit now and then, have run maybe 5 or 6 400m in the last 3 months. Best time is 64s, after a 3 lap warmup, average time is 67s.
My dad at 49 who was a national level rower in high school and excellent runner, even now trains regularly although mainly yoga, came down with me recently, and could just about run 85s.
Based on this, I am guessing the average american, - and I mean those who don't start walking at 200m- would finish at around 100s. No way the average would go faster than that.