I can see both sides. Maybe they ventured further out than usual, but after 10+ years of living in the same spot you would think they would have run in to any organized group that is within a day's walk from their cities. Granted people masquerading as zombies probably wouldn't have been obviously found, but this would have been a lot more plausible 10 years earlier in the plot line.
Either way, its a zombie flick where logic isn't always great.
I'm still assuming the Whisperers must have their own community with farming, etc. and somehow noticed Eugene and Rosita, grabbed their zombie herd and chased them down.