> but perhaps road walks
from my experience i would not reccommend it- a dog coming across an in heat bitch is pretty damn determined and you've got to remember your bitch will be trying to get mated, she won;t be helping you protect her. a dog can smell a bitch in heat from 2 miles away so be prepared for everything in your area.
i must say that having had both dogs and bitches over the years i would never take a bitch out during heat, its simply not safe for her or you. i also, and i certainly don;t mean this offensively, find it pretty selfish when people walk bitches in heat, your putting other peoples dogs in danger by doing so. dogs will do anything to get a bitch, including running away from owners, across roads, whatever- putting themselves in danger to meet that most powerful of urges and the dog owner has no idea that your girls in season so doesn;t keep the dog on lead etc.
your lab cross should be fine with her i think but hopefully someone more experienced with breeding will come along