Do not underestimate the abilities of a young shepherd in Judea in biblical times. He constantly had to protect his flock of sheep and goats from attacks by brown bears, leopards, and Asian lions, which still roamed the woodlands of the Judean foothills. Failing to do so would have got him in deep trouble with his father, so he had to become very adept at fending off such threats by whatever means possible—including devastating use of his slingshot.
As someone who now uses the Double Teddy* routine for my regular exercise, I also have a much greater appreciation than ever before for the all-round fitness of shepherds: walking for miles with a big staff (or better yet, two) provides a great all-body workout, and develops deceptively large muscle strength. David may have appeared modestly sized, but I don’t doubt he was supremely fit, agile, and strong.
Goliath may have been massive, but with all his armour, he couldn’t move quickly. David’s use of the slingshot was a direct extension of his shepherding experience in dealing with large carnivores, such as bears; it also heralded an early example of Israelite tactics of using brain against brawn, and ‘thinking outside the box’ to negate the advantage of larger conventional forces—something which he used repeatedly in his subsequent campaigns, as king, and repeated centuries later by the Maccabees against the Syrian Greeks, and in modern times against Arab armies (especially in the Six Day War of 1967).
*Walk softly, and carry two big sticks.