Restricted Sprint?

[AdamD]

If you are running away from something then eventually your fatigue is going to have some kind of bearing on your speed. The sprint should not run out but perhaps gradually slow down slightly so that the monster eventually gains on you if you fail to escape. The warning sign being your character's out-of-breath panting. Then, once you stop and are no longer in sight of the monster, your ability to sprint would be temporarily disabled until youve been given sufficient time to rejuvenate. It would maintain realism but also discourage players from abusing the sprint function.

It would have to be handled with subtlety though.

I disagree. The human body is capable of what you may think is impossible, thanks to adrenaline. If someone is running for his life, take for sure he will until his muscles explode (sometimes literally).
For example; a few years, in my town, a car ran down a kid and he went trapped under it. Then his mother, no more than 1.60 m and about 50 kg, was able to lift up the car so other people could take the kid away)