Told Im a natural

Possible semi-brag. So, last weekend, after not running for a year (I only run occasionally before, when i get a bit fat, etc) I installed a running app and went for a run. Tommorow my cousin asked did I went running and I mentioned that according to the app I run 4km in 17min. He then said that is impossible since people usually train for months to get those results. Is he overreacting and what is the average speed for an average person min per km? edit: 26M, 190cm, 80kg