Cheetahs. They're pretty fast, right? There's no way a human could ever catch one, right? Wrong.
Humans are evidently not the fastest creatures to roam this planet, but we are pretty good at getting anywhere we want to be, no matter how long it takes us to get there. Many creatures rely on hind legs to thrust them forward - have you ever noticed how a cat's or a dog's back legs are practically springs? They push themselves forward, accelerating incredibly rapidly, but at the expense of quite a bit of energy - and all that muscle movement in the abdomen can make it difficult to breathe effectively in the midst of a sprint. Humans, though? We just fall over. Seriously. Try to start walking forward, but then don't put your foot down - actually, don't, I don't want to be liable for any injuries. But the thing is, humans walk by simply falling forward, and then relying on that forward momentum to help lift us back up again. Talk about efficiency! You might not be able to run a mile in a minute, but you could certainly walk several before you run out of breath.
So, about that cheetah. How could you ever catch it, you might ask? Well that's simple! Just keep walking (or, in Dory's case, swimming)! Eventually, the cheetah will tire out, and be forced to rest. When you catch up to it, it won't be able to run any more!
Now, this is all relatively related to something that's more of an engineering term - mechanical advantage. In the simplest of terms, you can use the same amount of energy to do two things: go really far, or go really fast. The more Physics-y relationship is Work = Force * Distance. For the same amount of work, you can either maintain a large force over a short distance, or maintain minimal force over a large distance. Humans have opted for the minimal force over a large distance - most of the work we do is actually just to keep us from falling to the ground. Gravity basically just uses our legs as a lever to do the rest.