Powerwalking A Half Marathon

What Pondee said. I've run several races (Run/walk intervals at 4min run/2 min walk) and have been passed by walkers. I marvel at what they can do! If you average 13-14:30 a mile you should be able to complete the 13.1 in close to 3 hours. Race-day atmosphere will give you a boost of adrenaline. Good luck!!

At 13:00 to 14:00, your going to be beating runners.
Many halfs and fulls have pacers and pace groups. Just start in the right area, if it exists

Just weighing in to say that in general runners are a pretty friendly and encouraging group, no matter how fast you are. I've gone to events as a 20 minute mile morbidly obese walker and 9 minute mile average-weight runner and everywhere in between and never had anyone be rude. Nearly everyone is there to beat their own personal best and share a sport we all enjoy and others' speed won't really matter (except maybe for the medal contenders... they want to beat out the competition!). If you are walking in the 13 minute range, you will likely be surrounded by people who are doing a slow jog or run/walk intervals. They won't look down on you but will be impressed and slightly insecure that you are hanging with them without ever switching out of a walk.

As far as etiquette goes, I'd say just to pick the event well and make sure you are able to finish in the time allotted... race managers get annoyed at people who go so slower than posted time limits, blocking their ability to re-open roads safely at the time they are supposed to. Or that keep volunteers at the water stations long beyond their expected shift. Sounds like you are fine in terms of your event choice and can be under the posted time.

The other HUGE etiquette thing is to not block the faster people... that's already been mentioned. It was so frustrating at my last half marathon when we had clearly marked corrals and I was in the 10 minute corral... we took off running about that speed but within a tenth of a mile there were a lot of people just stopping and walking at a 20ish minute pace right in the middle of the road. I had to brake and find a way around them, it was annoying. NO way those people were finishing at the time they stated on their registration form and there were ample chances to change to a slower corral in a situation where they were overly optimistic on registration day and training didn't go as planned....so I don't get it. Maybe they don't understand that their official time is the chip time, not the gun time, so they think they can get some advantage by starting close to the front??? Anyway... stay with similarly paced people and if you need to slow down, don't do it abruptly in the midst of a crowd of people moving quickly and definitely don't do it while in the inside of a turn where people are running tangents to reduce their time. Since you are a walker, this last part (about slowing down) probably won't apply to you since you'll be going a pretty consistent pace. The people doing walking/running intervals might take off quickly and then stop abruptly, so watch out you don't plow into them.