Both are good depending on the purpose! When it comes to diseases like diabetes, Blood Pressure & heart related problems, both walking & running are equally beneficial. However if you want to lose weight, running is much better option than walking because in running 2.5 times more energy is consumed than walking. So, runners lose more weight than walkers. In any case make exercise a daily habit as after-all just 15 minutes of exercise a day can add 3 more years to your life!