You are right: most au pairs prefer large cities to the countryside. Thus, your choice will be slightly reduced. However, the most important thing for you is to find a matching au pair.
In order to help you with your search, we would like to give you a few tips:
Search for an au pair who likes sports and is active, thus being able to keep himself/herself busy. Somebody, who possibly likes outdoor activities. An au pair, who loves to go shopping or to the cinema everyday, would most likely not be happy in your home.
Emphasize the advantages of life in the country and of your own family in the free texts of your profile on AuPairWorld. Think about which activities are offered close to your home. Are there riding opportunities or a nice hiking trail?
For your au pair it would be a great offer to be allowed to use your car for the way to his/her language course. What about public transport in your area? Are there buses or trains regularly? All these points would be an additional plus, which you should definitlely highlight in your profile.
Depending on the au pair's country of origin, the meaning of "living in the country" may differ. In Canada or Finland this would mean that most families live quite far afield. In Germany, which is densely populated, "living in the country" may mean something entirely different.
This applies to your situation and to the fact that your au pair would have to drive 14 kilometres to attend his/her language course. This is still reasonable, though. After all, the most important thing is that your au pair feels happy with your family and that you both get along well with each other.