"The most common type of Infected, Walkers move at a slow but steady pace with ruthless determination. They are not difficult to kill or evade in small numbers, but in large groups they can be lethal..."
When facing multiple walkers, the 'Fall Back' option can make things much easier. This will often lead to walkers on the same level of the initiative grid to join into a Horde, allowing you to kill several in a single combat turn rather than having to take them on one at a time.