The undead horde is numerous, never tires, and never stops roaming the city in search of fresh recruits. Often enough, you will run into or be spotted by a larger group of undead that you cannot defeat with a few pistol rounds or a swing of your baseball bat - this is the point where it is healthier to turn around and run. Here are some basic rules:
1. Do not run where you came from.Edit
Zombies may have picked up your scent trail, following your tracks like a disorientated mob of fangirls. Soon enough you may find yourself trapped in a street between the mob you ran away from in the first place and the shamblers following you.
2. Keep an eye on your stamina and sleep.Edit
The un command is great in order to put some distance between you and your chasers immediately or to bolt around a lonely shambler, but you should save up some of your stamina in order to be able to climb cars.
Being sleepy will make you slower, making it almost impossible to escape the horde. Ideally, carry some blue pills for a long run.
3. Use barriers to your advantage.Edit
Sometimes, burning car wrecks are arranged in u-formations or act as a barricade between two buildings. As shamblers, fresh zombies and skeletons cannot climb or intelligently circumvent obstacles, they will be unable to follow you or your scent trail if you jump over any obstacle they cannot either break or circumvent with one step. Note that this will not shake zombie masters, as they have the ability to climb.
4. Run as far as you can.Edit
Bolting around the next corner is hardly enough to shake anything stronger than a skeleton. You need to put enough distance between you and them in order for them to loose your scent. Escaping through rainy streets or the sewers greatly aids in covering up your tracks.