The thing is, this isn't a video game, you can't just change a variable to slow the zombies down and balance things. Real people probably won't have much fun being shambling zombies unless there's a pretty good mob of them right off the bat, and even that might be testing their patience.
thats just it,since it isnt a videogame,you can pretty much set it up however you want.having the zombies being able to run means you dont have a chance to react,youll never be able to stop them,and you willl inevitably have the same outcome every time you play,people will get tired of trying,and youll never play that game again.theres videos on youtube of zombie games,that started out with just a few slow zombies,and the zombies still won.
that said,i do agree with you on the mob of zombies part.it all depends on the size of his group though.(the video i watched had about 50 people on the field). maybe also try having more of an objective than moving from point to point.if the op only has a small group,make half of them zombies,and split the other half into teams.both teams start off with a gbb or shotgun,and 1 mag.from the starting point(on opposite corners of the field,but both on the same side of the field),both teams must find a small cache of weapons and ammo(maybe 2 aegs,2 mid caps each,and 2 more gbb mags or shotgun shells).from there,the teams must meet up at a designated spot on the field.from there,they must make it to whatever kind of escape zone they want to set up.zombies can free roam the field,and are encouraged to spread out and actually hunt the humans,that way both team have a real possibility of sneaking up on each other.(maybe make a rule also that zombies are not allowed to "camp")you can break it up into rounds,meaning at each checkpoint,you stop for a couple minutes(water break),and cut the field size down,they cannot go back into the area they have already cleared.players turned zombies stay zombies until the end.you can make it more challenging for either team by calling headshots only,or any hits count.i would still stick with the respawning zombies though,but maybe only do the 10 second thing.as for turning humans into zombies,you could have the humans wearing flags,and the zombies have to actually pull the flag to turn someone.turning can be instant,or have a delay(instant if the humans are doing good,so if someone if the human turns,they can instantly grab their teammate.
when you really start thinking about it,the possibilities are endless...