Good question. It all has to do with distance your going. When I'm walking quarter mile sprints (interval training) I walk about 9:20 per mile pace. My best mile pace right now is right around 10 min per mile and my 5K is around 35 min. My best half this year was 2:45 which comes to around 12:30 per mile for the half and 11: for the 5K.
When you are training first think good technique. Walk in a straight line, arm swing more to the back than the front and walk straight up and not bent forward.
Do a minimum of three walks of at least 30 min a week and try to find time for 4 to 5 times a week. When you have a good routine down then do one of your walks longer then the others, walk that one for an hour instead of a half hour. Then make one of your walks at a faster pace than the others. Add 10 or 15 min to your long walk every other week till you have it to 2 hours.
When you can walk for 2 hours briskly and feel at the end that you have plenty left you are ready to walk a half marathon.
Dave