If you take the average american man, aged 20-30, at a healthy weight/BMI but not rail thin, I think that with a few months of running many would run under 20min. By many I mean like 30% within, say, 4 months.
That was the position I was in when I began running, age 18. skinny/skinny-fat but untrained physically. had never shown talent for running during my previous sport history (soccer, baseball, middle school track). I ran a 6:45ish mile in 7th grade when we did some jogging during PE class, plus I was playing soccer.
Pretty much off the couch I could run 9-10min pace for 5k, then after a couple months of doing a couple 2mi runs per week I was down to 23min. after about one month of 25 mpw I did 20:00 for 5k. however, that was starting after I had trained for a sprint triathlon a few months earlier and then did some cycling for a couple months, so I was fitter than average for sure and no longer overweight at all.