I was running with my wife at an indoor track one evening when about twenty army reservists showed up to do a time trial of about 1.5 miles. My wife, who is 50, was running 5 miles at about a 9:15 mile pace. I was running longer at around 8:00 pace.
There was only one guy who ever passed me (eight laps per mile), two that passed my wife, and we lapped all others, many multiple times. Some got cramps and were limping by the end. I'd say ages were from 25 to about 40. One guy who had one lap left decided to stay ahead of me the rest of the 200 m, but he only could manage about 120 m before he had to ease up and let me go by at my blistering 8:00 pace.
I'm guessing the average mile time for an untrained 30 yr old - would be no better than 10:00- possibly 11:00. This was a small sample size, but It taught me that most of those who never train to run truly suck at running.