A fast pulsing feeling in the stomach during walking is usually the normal abdominal pulse becoming more noticeable when your heart rate increases with activity. Try slowing your pace and building up gradually so your body adjusts better, and avoid walking on a full stomach, as this can make the sensation more obvious. Stay well hydrated and keep your breathing steady while walking to reduce awareness of the pulse, and maintain regular exercise so your body gets used to activity over time. Pay attention if the pulsing feels very strong or painful, or if it happens even when you are resting. Consult a doctor to rule out any underlying issue if it concerns you.