You're conflating the mechanical in-game movement speed (which is mostly the same between characters) and real-life movement speed (which is highly variable, and quite unlikely to be the same).
I was just watching a review of a basketball game today, and in one play where the team got a turnover, players started running down the court. The big, tall, slow player barely got 3 steps in while the short fast player traveled 3-4 times the distance in those couple of seconds, going from well behind to well ahead. The speed wasn't really even close. And both players are professional athletes.
I've also seen a YouTube video where a guy was talking with a professional sprinter, and wanted to see how well he'd do in comparison. The guy was healthy and fit, but the difference in speed was just ludicrous.
Some people are just faster, and some are better trained (training for how to successfully retreat is a big thing in the military), and that's most of how people successfully retreat (or chase down a fleeing person, from the other side). And if speeds are actually similar, then it's a matter of endurance.
But the game abstraction of speed strips all of that away, and it becomes that largely unwinnable mess that several people have already described.