If she has a fear of traffic (a mostly healthy fear), look for long trails and quiet roads.

One thing I've found to be an issue is the length of ride. It may be because my girlfriend's bikes don't fit her well, but she often doesn't want to go as far as I'd like to. So you might also have a disagreement about mileage. If it's not as important, you might not mind compromising. The longest ride we've done is 54 miles, I might like to go further. Another thing, not everyone likes to camp. Check for inns and B&Bs along the way.