[ssendur]

you use present progressive tense( be + verb +ing) for two reasons: 1) to explain progression 2) to tell near future(which is not planned before)
so you dont want to tell that you run to the store generally(you might do that or not), the thing you want to tell is that you will go to the store soon or you are on your way of going to the store. my first language is not english so i am just giving a try for the topics. my english is not so good, so mines are just suggestions.