Ok that's all well and good...
So what if F is running away and S is doing the chasing?
Then you've got a faster person running away from a slower person, and, barring the intervention of fate points, clever maneuvers / declarations, or a sequence of really lopsided rolls, the faster person will simply get away. I don't see a problem with that; if you're a human trying to run down a cheetah on a straightaway, you're going to lose.
There are, of course, all sorts of maneuvers you can try - from finding shortcuts, to the patch of gravel that other people have mentioned, etc. But if you're person S, trying to chase down person F, you can't win in a straight contest, and will have to either get creative or accept defeat.