Because walkers must use all 4 phases of gait, bodyweight is distributed more evenly over the entire plantar (bottom) portion of the foot and the gait cycle takes longer to complete, thereby putting less stress on the joints, tendons and ligaments.
Depending on technique, runners may not use all phases of gait, therefore the foot has to take more weight in a shorter period of time over an abbreviated gait cycle. Some runners are midfoot strikers which means that they do not strike the heel at all, while some are forefoot strikers where all of the weight has to be distributed over one phase and loaded onto one foot. Adding to this force is the fact that runners exert up to 4 times the load on the foot as the result of momentum generated during their stride. As a general rule walkers can use running shoes but runners should not wear walking shoes.