Only power walk if you're grinding up some gnarly hills. Otherwise, run easy as if it were any other standard long run.
If possible, run outside for an hour or so and then hit your treadmill with some power walking at 12-15% for 30-90 minutes. Follow that with an hour or so outside again. If you don't have a treadmill, run to the gym and do the same. If the gym isn't within running distance, drive there and run from the gym and do the same thing.
Basically, it would be great to get used to a combo of running and walking in a long workout because, unless you're an elite runner, you're going to be doing a lot of walking for PP. However, if you don't have good hills in your area, just run long as usual and include some treadmill walking. If you do have good hills, hike the hell out of those inclines, bomb the declines, and run steady and easy on the flats.