Pulling out of an encounter gone bad should always be an option unless the PC's are confined to some kind of "cage match" battle. The difficulties of doing so can vary considerably depending on who or what the party is fighting. There are some factors that may be of influence to the difficulty level.
1) Intelligence:
Is the party fighting a dumb hungry beast, a mindless guardian automaton, or guards of a somewhat intelligent variety?
A beast might be delayed by throwing down food during the chase. If a critter is hungry then it will probably stop for something edible that isn't currently trying to hurt it.
Treasure is another slowing technique that intelligent but dimwittted creatures like kobolds might fall for. Monsters that recognize the value of treasure might pick up shiny baubles.
Mindless constructs might have orders not to go to far from what they are guarding.
2) Motivation:
This will be situational. Guards of at least something appoaching human intelligence that are defending a secret base will care more about making sure intruders don't get away than any other concern (considering reporting failure in this regard could earn a death sentence from their leader!)
These types of enemies will be the most tenacious pursuers and the hardest to get away from assuming the party doesn't seriously outclass them in speed/evasion.
3) Captivity:
In many cases the more organized and intelligent the enemy is the more likely they will take an interest in captives. A hungry dire wolf just wants a meal and is unlikely to be reasoned with. Guards might have orders to try and capture intruders because the leaders may want to find out if anyone else knows about their secret lair before killing the captives.
Largely this means that the pursuers that are the hardest to get away from are also the most likely to take captives. This can be used to help mitigate TPKs.
Intelligence will also determine how difficult escape will be for the PCs if they do get captured. Getting captured by a bunch of stupid orcs might result in the PC's being sent out to work and given tools/weapons and guarded by lazy/inattentive taskmasters *
(sorry Forest Oracle- handing a dwarf an AXE and commanding that he chop wood guarded by a single orc is the epitome of captive mismanagement

)
Savvy, competent operations might completely strip and bind the captives, blindfold, gag and separate them to prohibit communication and limit sensory input, and interrogate them separately- the kinds of things that PCs might do to captives that they deem very dangerous.
Escape from these types should be difficult but not impossible unless the campaign is ending.
So, just keep in mind who or what the PC's are running from and you can use common sense to help determine how easy escape might be.