We aren't as vulnerable as movies make us out to be. Humans evolved to run over distances, and can even theoretically beat a horse in a marathon. Large animals take longer to build up speed, and are gassed in no time. And large animals aren't built like a cheetah or greyhound, instantly running down prey. A T-Rex would be something closer to a Saint Bernard trying to catch a tennis ball rolling down a hill and giving up on it after ten yards. A cranky ol' T-Rex huffing and puffing after Jeff Goldblum as he casually hustles away would be a very different movie, but ... not necessarily bad?
You can try this "leaving in a hurry" defense against a lot of big animals still alive today. The size-to-endurance ratio is a consistent principle of biology in elephants, hippos, and rhinos. Large size is a disadvantage for predators. Smaller, tastier prey like us have a huge metabolic advantage. T-Rex's prey was believed to consist of other large, bumbling dinosaurs, like hadrosaurs or triceratops, or maybe even each other. The point is, a T-Rex chasing after a single human would be like you chasing a speeding taxi because you left a French fry inside it.