Let me start off by saying you are going to have to start running soon if you want to complete a half marathon run. Am I correct in assuming that you do not run at all now? I have done a half marathon walk and walk at a very brisk pace. I do not do the racewalking thing. Your legs, hips and other body parts need to get used to the increased activity. It is the same for running. You have to condition your body to be able to run that far. Your lung capacity and breathing is a major deal. I would suggest starting slow and working your way up. I would look at the half marathon race you are going to do and see what the time limit is to complete it. There are quite a few that have 3 1/2 or 4 hour limits. If you cannot complete it in that time you are either asked to leave the course or have to deal with vehicle traffic. The half marathon walk that I did had a 5 hour limit. I did it in 2 hours and 47 minutes, which is a 12:46 per mile pace. The 5 hour time limit is a 22 minute per mile pace. The runners were on the course at the same time and I saw alot of them walking portions of the race. You need to judge where you are currently in your training.

Running a long race successfully is much more than just breathing and strong leg muscles. If you plan on running or walking a 1\2 marathon you'll need to 'practice' by actually running and/or walking in order to know how your mind & body holds up to the mental and physical stress. You need to know if your shoes & clothing feels comfortable; is your hydration & nutrition adequate; how your feet & toenails withstand several hours of pounding the pavement; what type of music (or no music) do you prefer; do you have any weak points in your muscles, back & joints that support your body while running distances (in which case you can head off serious injury or build up weak points in time for the race). I believe successfully finishing a 1\2 marathon upright, in time and (most important) uninjured without practicing by running is unwise.