Wow, I haven't seen one of these Zombie Walks in a while...
An important thing to remember is, (depending on the climate of where you live), have a water bottle with you to keep hydrated. I know it might ruin the zombie facade, but believe me, getting a dehydration headache halfway-through SUCKS.
I also find that red fabric paint mixed with a small amount of black fabric paint makes pretty realistic, dry splattered blood. If it's a spray can of fabric paint, push the sprayer down only halfway to get the splattered I-just-ate-someone look.
Don't wear your fancy shoes... (unless they're super comfortable)
I'd go with plain black running shoes.
That's about all I can offer concerning this...
Hope you have fun!