We must remember that David and his men had travelled not only from Ziklag to Besor, but before that also from Aphek to Ziklag. They were all tired and those who were not so strong/accustomed to travelling would have struggled. Many of them were likely travelling on foot and carrying heavy equipment.
9 So David set out, and the six hundred men who were with him, and they came to the brook Besor, where those who were left behind stayed.